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  1. 1. CHAPTER 1 Introduction to NetworkingObjectivesUpon completion of this chapter, you should be able to answer the following questions: ■ What are the requirements for an Internet ■ What are the Base 2, Base 10, and Base 16 connection? number systems? ■ What are the major components of a personal ■ How do you perform 8-bit-binary-to-decimal computer (PC)? and decimal-to-8-bit-binary conversions? ■ What procedures are used to install and trou- ■ How do you perform simple conversions bleshoot network interface cards (NICs) and between decimal, binary, and hexadecimal modems? numbers? ■ What basic testing procedures are used to test ■ How are IP addresses and network masks the Internet connection? represented in binary form? ■ What are the features of web browsers and ■ How are IP addresses and network masks plug-ins? represented in decimal form?Key TermsThis chapter uses the following key terms. You can find the definitions in the Glossary:Internet page 4 parallel port page 10enterprise network page 4 serial port page 10Internet service provider (ISP) page 6 mouse port page 10personal computers (PCs) page 7 keyboard port page 10central processing unit (CPU) page 8 Universal Serial Bus (USB) port page 10random-access memory (RAM) page 9 expansion slots page 10disk drive page 9 network interface card (NIC) page 11hard disk page 9 video card page 11input/output devices (I/O) page 9 sound card page 11motherboard page 9 jack page 14memory chip page 9 local-area network (LAN) page 14 continues
  2. 2. 2 Networking Basics CCNA 1 Companion GuidecontinuedEthernet page 14 web browser page 26plug-and-play page 16 web servers page 26bits per second (bps) page 17 binary digit (bit) page 26networking devices page 17 byte page 26Media Access Control (MAC) address page 19 plug-ins page 28servers page 20 Transmission Control Protocol (TCP) page 28media page 21 IP address page 31modems page 22 dotted decimal page 31digital subscriber line (DSL) page 23 Universal Resource Locator (URL) page 35standards page 25 default gateway page 37protocols page 25 ping page 39Hypertext Transfer Protocol (HTTP) page 25 tracert page 42Internet Protocol (IP) page 25 ASCII page 47protocol suite page 25 decimal numbering (Base 10) page 48Transmission Control Protocol/Internet Protocol (TCP/IP) binary numbering (Base 2) page 49 page 25 hexadecimal numbering (Base 16) page 59
  3. 3. Chapter 1: Introduction to Networking 3xxxxxxxxxxxxxxxxxxx This chapter introduces the basic concepts and components of modern computer networks, Presented by: including the basics of the TCP/IP protocol suite, upon which most modern networks are built. This chapter also covers some of the related binary, decimal, and hexadecimal math that is required to examine the details of how computer networks work. This chapter, along with Chapter 2, “Networking Fundamentals,” provides an overview of many of the topics related to computer networking, introduces many terms, and provides a solid foundation before you get into more detailed subjects in later chapters. Connecting to Networks and the Internet The Networking Academy course that you are (likely) taking when using this book may be your first formal introduction to the world of computer networking. However, today, most people have grown up with networks and networking as part of the overall culture of the developed world. As a result, most people start this course and book with some opinions about what a net- work really is and what the Internet is. This section formally defines a network. It also defines the basic concepts and terms behind one special and important network: the Internet. What’s a Network? To formally begin your networking journey, you need to start forming a more detailed and spe- cific answer to the question “What’s a network?” Assuming that you took the time and effort to register for the Cisco Networking Academy Program CCNA 1 course, which is a basic net- working course, you probably already have some opinions about the answer to this question. This section begins to answer the question. First, consider the following formal, but general, definition of a computer network: A combination of computer hardware, cabling, network devices, and computer software used together to allow computers to communicate with each other. The goal of any computer network is to allow multiple computers to communicate. The type of communication can be as varied as the type of conversations you might have throughout the course of a day. For example, the communication might be a download of an MP3 audio file for your MP3 player; using a web browser to check your instructor’s web page to see what assign- ments and tests might be coming up; checking the latest sports scores; using an instant-messaging service, such as AOL Instant Messenger (AIM), to send text messages to a friend; or writing an e-mail and sending it to a business associate. This chapter starts the process of closely looking at the four networking components mentioned in the formal definition: computer hardware, computer software, cabling, and networking devices. Before you look at each component, however, it is helpful to think about some exam- ples of networks. Reproduced from the book Networking Basics CCNA 1 Companion Guide (Cisco Networking Academy Program). Copyright© 2006, Cisco Press. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses.
  4. 4. 4 Networking Basics CCNA 1 Companion Guide A Small Network: Two PCs and One Cable You can create a simple network with two computers and a cable. Although it’s not a terribly impressive network, such a network does occasionally serve a good purpose in real life, as well as being useful for discussing networking and learning some basic skills in classroom labs. Figure 1-1 shows such a network. Figure 1-1 A Two-PC, One-Cable Network A B Figure 1-1 shows two computers, A and B, and a line that represents a networking cable. Implemented properly, this small network allows computers A and B to communicate. (That “implemented properly” phrase is simply a way to ignore the details you will learn over the coming months. More on that is covered in upcoming chapters.) This network certainly meets the formal definition for a computer network because multiple computers can communicate. Although this network might seem small, small networks do have some useful purposes. For example, when you download a song to your PC and copy the song to an MP3 player over a cable, you have effectively created a small network. Another example of a small network is when two people with laptops attend the same meeting and use wireless to exchange files while sitting in the meeting. A Very Large Network: The Internet Consider a network that is the opposite of the simple network shown in Figure 1-1: the Internet. The Internet is somewhat challenging to define because it means many different things to so many people. From one perspective, the Internet is a very large, global network that allows almost every computer on the planet to communicate with the other computers on the planet. Not only is it a network in the formal sense, the communication it enables worldwide, across cultures and political boundaries, has fundamentally changed the world as we know it. Under close examination, however, the Internet isn’t a network at all. It’s really a bunch of interconnected networks. In fact, that’s how it got its name: Internet is short for interconnected networks. Figure 1-2 depicts part of the Internet. All the pieces of Figure 1-2 create the Internet. First, on the left, two enterprise networks are shown: Retailer1 and Supplier1. The term enterprise network refers to a network built by one company, one government institution, one school system, or any other entity. In this case, these two companies hired network engineers to plan and implement a network that these companies’ employees can use. At that point, the companies can carry on business communications between computers inside their respective companies.
  5. 5. Chapter 1: Introduction to Networking 5Figure 1-2 Internet Retailer1 Customer At Home Web Browser Web Server Retailer1 The Internet Supplier1Besides communicating inside their respective companies, these two companies need to com-municate with each other. Retailer1 needs to exchange information with its supplier, Supplier1.(For example, the retailer might simply need to order additional stock to fill its stores’ shelves.)So, both Retailer1 and Supplier1 connect to the Internet, which allows the computers in the twocompanies to exchange information, such as orders and invoices, check on shipping and prod-uct availability, and the like.Retailer1 also needs to communicate with its customers. Because Retailer1 sells consumerproducts, these consumers need to be able to get to Retailer1’s website, which is located insideRetailer1’s enterprise network. Therefore, Retailer1 has a second reason to connect to theInternet.Next, potential customers also need to connect to the Internet. In Figure 1-2, the Retailer1 cus-tomer sits at home and uses a home computer and an Internet connection. After she’s connectedto the Internet, the customer can browse Retailer1’s website, find products, order the products,pay via a credit card, and so on.The Internet includes literally hundreds of thousands of enterprise networks, hundreds of mil-lions of home users, and a mysterious cloud in the middle of Figure 1-2. When drawing figuresof computer networks, if a portion of the network contains details that are not important to aparticular discussion, that part of the network is typically represented as a cloud. Figure 1-2 isno exception. It shows the “Internet” as a big cloud without any details. Figure 1-3 removes thecloud, shows some details, and shows some other clouds.
  6. 6. 6 Networking Basics CCNA 1 Companion Guide Figure 1-3 Internet: A Closer Look Company1 Customer At Home Web Retailer1 Browser Web Server ISP1 ISP3 Supplier1 ISP2 The core of the Internet is not one entity, but many. To create the Internet, a company called an Internet service provider (ISP) creates a network. An ISP then sells its services to businesses and individuals, with the most basic service being the ability for the customers’ computers to send and receive data to and from any other computer on the Internet. To provide this basic overall service, an ISP must provide a customer with two things: ■ A connection between an enterprise network, or a home user, and the ISP’s network ■ Connections between the ISP’s network and every other part of the Internet In Figure 1-3, three different ISPs supply a network connection to their respective customers. The home user, Retailer1, and Supplier1 each pay a fee, typically monthly, to their respective ISPs for the right to connect to that ISP. However, they do not pay money to the other two ISPs shown in the figure. For example, Retailer1 uses ISP1, so Retailer1 pays only ISP1 for its Internet service. Such agreements allow any company or individual to connect to an ISP, and it provides competition to keep prices more reasonable. The ISPs must connect to each other so that they can forward traffic to all parts of the Internet. Figure 1-3 shows a direct line, which represents some networking cables, between two pairs of ISPs. The ISPs must have some path to each other so they can forward traffic between their respective customers, fulfilling their promise to connect their customers to the rest of the Internet. ISPs do not need a direct connection to all other ISPs to meet the requirement of being able to reach all parts of the Internet. For example, ISP2 and ISP3 might need to send data between
  7. 7. Chapter 1: Introduction to Networking 7each other for some of their customers. To do so, they send it through ISP1. As long as somepath exists so all ISPs can reach all other ISPs in the world by using one or more different ISPs,the requirement for complete connectivity to the Internet is accomplished. NotePerspectives on Networks of Different Sizes Figure 1-3 shows severalComparing the simple network of Figure 1-1 with the Internet in Figure 1-3 shows how differ- cylindrical icons thatent networks can be, particularly in size. In fact, many individual enterprise networks connected resemble hockey the Internet have more than 10,000 computers connected to them, in hundreds of locations. They represent a network- ing device called a router.These types of enterprise networks are complex in and of themselves. Also, home users might Later chapters of this book,have multiple computers connected to a home network that’s connected to the Internet. and major portions of the other three courses of theInterestingly, as you dig deeper into how networks work, you can see that many of the network- Networking Academying concepts covered in this class are used in small, medium, and large networks—even the CCNA curriculum, expound upon the purposeInternet. Certainly, the larger the network, the more work and effort it takes to successfully and inner workings ofimplement the network. However, that complexity—and the requirement for more effort and to successfully implement networks—is actually a good thing because it means morejobs, more variety in those jobs, and more opportunity.Next, you closely look at some network components and begin to understand how networkengineers can construct a network.Network ComponentsThe people who create a computer network, referred to as network engineers, create networksby combining the four things mentioned in the formal definition of a network: ■ Computer hardware (including NICs) ■ Cables ■ Networking devices ■ Computer softwareThis section closely looks at the first three of these networking components. Networkingsoftware is covered later in this chapter in the section “TCP/IP Protocol Suite and TCP/IP Software.”Computer HardwareComputers come in many shapes, sizes, and types. However, the vast majority of people usecomputers that are best categorized as personal computers. Personal computers (PCs) are com-puters that are specifically designed to be used by a single person at a time.Although some knowledge of the basics of PCs is important for this course, you do not needdetailed knowledge of PCs to do well in this course. If you are new to computers or if you wantfurther background on PCs, take the HP IT Essentials I: PC Hardware and Software course or
  8. 8. 8 Networking Basics CCNA 1 Companion Guide read the book HP IT Essentials I: PC Hardware and Software Companion Guide (published by Cisco Press). The next several subsections cover the most commonly discussed PC components. General Types of PC Components From a basic perspective, a PC has the following components: ■ Processor (also called a central processing unit [CPU])—A computer processor, or CPU, acts as a computer’s brain. A CPU’s job is to process, or think about, what the computer is trying to do. (Figure 1-4 shows a picture of a CPU.) The CPU’s job includes many things, such as the following: ■ Creating the image that is displayed on the computer’s screen ■ Taking input from the keyboard or mouse ■ Sending data over a network ■ Processing data for software running on the computer Figure 1-4 CPU ■ Microprocessor—A silicon chip that contains a CPU. A typical PC has several micro- processors, including the main CPU. ■ Temporary memory (also called random-access memory [RAM])—The processor needs memory in which to work on things. RAM is the computer equivalent of the papers and notes you might keep on your desk when studying. The CPU can quickly and easily access the data stored in RAM, and that data typically pertains to something the PC is actively processing. Note that the contents of RAM are lost when the computer is powered off. ■ Read-only memory (ROM)—ROM is a type of computer memory in which data has been prerecorded. After data has been written onto a ROM chip, it cannot be removed; it can only be read. (PCs can re-record information into another type of ROM, called electronically
  9. 9. Chapter 1: Introduction to Networking 9 erasable programmable read-only memory [EEPROM]. The basic input/output system [BIOS] in most PCs is stored in EEPROM.) ■ Permanent memory (such as disks)—Computers need long-term memory to store data that might be needed later, even after a computer is powered off. Permanent memory typi- cally consists of a type of device called a disk drive or hard disk. ■ Input/output devices (I/O)—To interact with humans, the computer must be able to know what the human wants it to do and provide the information to the human. Humans tell a computer what to do by manipulating an input device. For example, this occurs when the human types on the PC keyboard or moves/clicks with a mouse. For output, the computer uses a video display, audio speakers, and printers.With these components, a PC can take input from the human, possibly gather data from thedisk drives into RAM, process the data with the CPU, and provide the results through one ofthe output devices.MotherboardThe PC motherboard holds many of the PC’s most important components. The motherboardis a flat piece of plastic called a circuit board. Circuit board material is designed to be a goodplace to physically attach microprocessor chips, such as the CPU and RAM, and connect thecomponents with wires and other hardware. The following list details some of the mother-board’s individual components: ■ Printed circuit board (PCB)—A thin plate on which chips (integrated circuits) and other electronic components are placed. Examples include the motherboard and various expan- sion adapters. ■ Transistor—A device that amplifies a signal or opens and closes a circuit. Microprocessors can have millions of transistors. ■ Integrated circuit (IC)—A device made of semiconductor material. It contains many tran- sistors and performs a specific task. The primary IC on the motherboard is the CPU. ICs are often called chips. ■ Memory chips—Another name for RAM, memory chips are Integrated circuits whose pri- mary purpose is to be used to temporarily store information that is processed by the CPU. ■ Resistor—An electrical component that is made of material that opposes the flow of elec- tric current. ■ Capacitor—An electronic component that stores energy in the form of an electrostatic field. It consists of two conducting metal plates separated by insulating material. ■ Connector—A port or interface that a cable plugs into. Examples include serial, parallel, USB, and disk drive interfaces. ■ Light emitting diode (LED)—A semiconductor device that emits light when a current passes through it. LEDs are commonly used as indicator lights.
  10. 10. 10 Networking Basics CCNA 1 Companion Guide ■ Parallel port—An interface that can transfer more than 1 bit at a time. It connects external devices, such as printers. ■ Serial port—An interface used for serial communication in which only 1 bit is transmitted at a time. The serial port can connect to an external modem, plotter, or serial printer. It can also connect to networking devices, such as routers and switches, as a console connection. ■ Mouse port—A port designed for connecting a mouse to a PC. ■ Keyboard port—A port designed for connecting a keyboard to a PC. ■ Power connector—A connector that allows a power cord to be connected to the computer to give electrical power to the motherboard and other computer components. ■ Universal Serial Bus (USB) port—This interface lets peripheral devices, such as mice, modems, keyboards, scanners, and printers, be plugged and unplugged without resetting the system. PC manufacturers may one day quit building PCs with the older parallel and serial ports completely, instead using USB ports. ■ Firewire—A serial bus interface standard that offers high-speed communications and real- time data services. Expansion Slots and the PC Backplane For various reasons, some parts of a computer cannot be easily attached to the motherboard. For example, disk drives are too large to attach directly to the motherboard. However, these devices still need to be accessible to the motherboard. So, the motherboard includes connectorsNote that allow other parts, such as disk drives, to connect to the motherboard through a cable.Although a NIC is an Other necessary computer components might be connected to the motherboard, or might not beoptional component of a connected, at the discretion of the PC’s manufacturer. For example, the function provided by aPC, because most con- NIC, which is important to networking, might be included on the motherboard of a PC, or itsumers want networkaccess, most every new PC might not. (The upcoming section “Network Interface Cards” discusses NICs in more detail.)sold today has an integratedNIC. To allow for additional functions besides what is provided on a particular PC’s motherboard, PCs typically have the physical capability to accept expansion cards. These cards are built with the same general types of components as the motherboard: a circuit board, microprocessor chips, capacitors, and the like. However, these expansion cards typically fulfill a specific pur- pose. For example, if the motherboard does not include the same function as a NIC, a NIC canNote be added to the PC as an expansion card.Expansion cards are some- For expansion cards to useful, they must connect to the motherboard through the PC backplane.times called expansion The backplane is part of the motherboard that is designed as a place to allow the connection ofboards, or sometimes sim- the expansion cards. The backplane also provides several standardized plastic connectors, calledply boards or cards. expansion slots, into which expansion cards can be inserted. By connecting expansion cards into the backplane, the cards and the motherboard can communicate. Figure 1-5 shows a moth- erboard with the expansion slots in the lower-right part of the figure (the white vertically oriented rectangles).
  11. 11. Chapter 1: Introduction to Networking 11Figure 1-5 PC Motherboard and Expansion Slots Note The term bus is used to refer to a PC’s backplane. CPU Expansion SlotsThe following list identifies some of the more popular cards found in these expansion slots: ■ Network interface card (NIC)—An expansion board that provides a network communication connection to and from a PC. Many newer desktop and laptop computers have an Ethernet NIC built into the motherboard. (Ethernet is the most popular type of local-area network [LAN] in use today.) ■ Video card—A board that plugs into a PC to give it display capabilities. Video cards typically include onboard microprocessors and additional memory to speed up and enhance graphics display. ■ Sound card—An expansion board that handles all sound functions.Miscellaneous PC ComponentsThis section completes this chapter’s list of some of the PC’s components. Specifically, it definesa few different types of permanent storage options and the system unit and power supply: ■ CD-ROM drive—An optical drive that can read information from a CD-ROM. This can also be a compact disk read-write (CD-RW) drive, a digital video disk (DVD) drive, or a combination of all three in one drive. ■ Floppy disk drive—A device that can read and write to floppy disks (see Figure 1-6).
  12. 12. 12 Networking Basics CCNA 1 Companion Guide Figure 1-6 Floppy Disk Drive ■ Hard disk drive—A device that reads and writes data on a hard disk. This is the primary storage device in the computer. ■ System unit—The main component of the PC system. It includes the case, chassis, power supply, microprocessor, main memory, bus, expansion cards, disk drives (floppy, CD hard disk, and so on), and ports. The system unit does not include the keyboard, the monitor, or any other external devices connected to the computer. ■ Power supply—The component that supplies power to a computer by taking alternating current (AC) and converting it to 5 to 12 volts direct current (DC) to power the computer. Desktop Versus Laptop Laptop computers differ from desktop computers in that they can be easily transported and used. Laptops are generally smaller than desktops, with built-in video display and keyboard so that transporting it is convenient. In most cases, laptops weigh less than 10 pounds and have a battery that lasts several hours, so they can be used while traveling. Laptop expansion slots are called Personal Computer Memory Card International Association (PCMCIA) slots or PC card slots. Devices such as NICs, modems, hard drives, and other useful devices, usually the size of a thick credit card, insert into the PC card slot. Figure 1-7 shows a PC card for a wireless LAN (WLAN).
  13. 13. Chapter 1: Introduction to Networking 13Figure 1-7 PCMCIA Card Lab 1.1.2 PC Hardware In this lab, you become familiar with the basic peripheral components of a PC system and their connections, including network attachments. You examine the internal PC configuration and identify major components. You also observe the boot process for the Windows operating system (OS) and use the Control Panel to find information about the PC hardware.Network Interface CardsFor a PC to use a network, it must have some interface to the network cabling. PCs use networkinterface cards (NICs) to provide that interface. In fact, the name is somewhat self-descriptive:NICs are expansion cards that give a PC an interface to a network. Figure 1-8 shows a NIC.Figure 1-8 Ethernet NIC
  14. 14. 14 Networking Basics CCNA 1 Companion Guide When installing the card in a PC, the part of the card on the right side of Figure 1-8, which slightly sticks out, is the part that is inserted into the expansion slots shown in Figure 1-5. The silver part at the bottom of the figure is the part that can be seen from the outside of the PC. This side has an opening, typically called either a socket or a jack, into which the networking cable can be inserted. (A jack is simply a hole, with a standard shape, into which a cable can be easily inserted.) Figure 1-8 shows an Ethernet LAN NIC. The term LAN refers to a general type of network in which the distances between computers are relatively short—hence the phrase “local-area” in its name. Several types of LANs have been defined over the years, including Token Ring, Fiber Distributed Data Interface (FDDI), and Ethernet. Today, however, you are likely to use only Ethernet, in fact, the other two types of LANs have become so unpopular that it is difficult to find and purchase Token Ring or FDDI NICs. When purchasing a NIC, consider the following features: ■ LAN protocol—Ethernet, Token Ring, or FDDI. Today, Ethernet is used almost exclusive- ly, and Token Ring and FDDI are seldom used. (Token Ring and FDDI are introduced briefly in Chapter 6, “Ethernet Fundamentals.”) ■ Media supported—Twisted pair, coaxial, wireless, or fiber optic. ■ Bus support on the computer—PCI or Industry-Standard Architecture (ISA). Finding an Ethernet NIC on Your PC It might be an interesting exercise, at home or in the classroom, to look for an Ethernet NIC in your PC. To find one, look at the back of the PC where most of the connectors sit. Look for a jack, which is roughly rectangular in shape, like the dark rectangle on the side of the NIC shown in Figure 1-8. That’s the socket into which the cable is inserted. The cable attached to the NIC typically has a connector called an RJ-45 connector (RJ means registered jack). (The cable’s connector is simply the shaped plastic end of the cable.) The RJ-45 connector is shaped like the connector used on telephone cable for your home telephone, but it’s just a little wider.Note Figure 1-9 shows the shape of the RJ-45 jack and connectors to help you in your search.The connector on yourhome telephone cable is Note that although a NIC can be inserted into a PC’s expansion slot, many of the newer PCscalled an RJ-11 connector. sold today integrate the NIC’s functions onto the motherboard. So, when you look on your PC’s NIC, if you do not see the RJ-45 socket on any of the cards in the expansion slots, look at the other parts on the back of the computer; you might see the RJ-45 socket that’s connected to the motherboard.
  15. 15. Chapter 1: Introduction to Networking 15Figure 1-9 RJ-45 Jack and ConnectorsInstalling Ethernet NICsThese days, the physical installation of a NIC is relatively easy. The IT Essentials course coversthe details in more depth, but the process is extremely similar for any expansion card: Step 1 Shut down or power off the PC. How To Step 2 Disconnect the power cord from the PC. Step 3 Connect an antistatic strap to your wrist to protect the computer and NIC from your body’s static electricity. Step 4 Insert the NIC into the expansion slot. Step 5 Reassemble the PC and turn it on.After practicing a time or three, you should be able to easily insert the card. Figure 1-10 showsthe physical installation.Figure 1-10 Installing an Ethernet NICBesides the physical installation, the installer might or might not need skills relating to how toadd hardware to the computer’s OS, which is the software that controls the computer’s actions.(For example, Windows XP and Linux are both computer OSs used on PCs.) Oftentimes today,
  16. 16. 16 Networking Basics CCNA 1 Companion Guide you can physically install the NIC and turn the computer on, and the OS automatically adds the hardware. This process is called plug-and-play. If the plug-and-play process does not work, a successful NIC installation might require you to examine and change software settings using the software that comes with the NIC. This might require knowledge of how the NIC is configured in the OS and how to use the NIC diagnostics. It might also require that you have the skills to resolve hardware resource conflicts. The IT Essentials course covers this knowledge and skill. Network Cabling Basics For networks to work, the computers must have the capability to take the bits sitting in RAM on one computer and somehow send a copy of those bits into RAM on the other computer. For the process to work, the computer typically asks the NIC to send the bits over the cable that is connected to the NIC. So, the NIC must be connected to some form of transmission medium over which it can send the bits to the other computer. This section introduces the concept of transmission medium by using a specific example: the small Ethernet network shown in Figure 1-1, which uses an electrical Ethernet cable. As mentioned in this book’s Introduction, the chapters generally cover a slightly broader range of topics than the online curriculum. The small amounts of extra coverage provide you with better context or different ways to think about the same concepts. Occasionally, and particularly in this chapter, this book provides several pages of additional information on a few important topics. This section and the upcoming section “Networking Devices” are the first two sections in this chapter that take the discussions much deeper than what’s given in the CCNA course. You might appreciate the additional depth at various points in your reading. However, if you or your instructor prefer to skip over these deeper bits of additional coverage, feel free to skip forward to the section “Enterprise Networks and Home Internet Access.” The topics in this section are covered in more depth in Chapters 3, 4, 5, and 7, and the topics in the section “Networking Devices” are covered to some degree in Chapters 2, 5, 8, 9, and 10. Creating a Transmission Medium Using Cables To send data—bits—to another computer, the computers can use some physical medium over which electricity can be sent. In this case, the physical medium is typically a set of copper wires because copper easily conducts electricity. Because copper wires are brittle, the wires are usually wrapped in a colored plastic coating to help prevent them from breaking. The plastic coating also helps provide some electrical insulation, which is a property discussed in detail in Chapter 3, “Networking Media.”
  17. 17. Chapter 1: Introduction to Networking 17The LAN cable connected to the NIC contains multiple copper wires. NICs tend to use morethan one wire, with a typical Ethernet NIC using four wires. Additionally, the outer part of thecable, which is made of flexible plastic and other insulating materials, adds strength and protec-tion to the combined wires. The cable keeps the set of wires together for convenient use andphysically protects the wires.For the simple network in Figure 1-1 to work (for example, for PC B to send bits to PC A), PCB needs to be able to send electricity to PC A. To do so, the copper wire inside the cable needsto be physically touching something inside the NICs on each end of the cable to have a pathfrom PC B to PC A over which electricity can pass. Figure 1-11 shows the idea, with the NICsinside the PCs removed from the figure to more clearly show the details.Figure 1-11 Connecting the Copper Conductors in a Cable to the NICs A B View of NICs, Which Are Inside the PCs Cable NIC NIC ConnectorsBy connecting the NICs in the two PCs with the correct cable, the NICs now have a physicalpath between the two PCs over which electricity can flow. Now, they can use this electricalpath to transmit bits.The term bits per second (bps) often refers to the speed of network connections. Note that theunit is bits, not bytes. In real life, LANs typically run at much higher speeds, with a slow LANtransmitting at 10 million bits per second (megabits per second, or Mbps). The section “Namesfor the Rate at Which a Network Sends Data” describes the speeds at which bits can be sentover a networking cable.Networking DevicesThis chapter began with a formal definition of a network, which included components such ascomputer hardware and cabling. This chapter so far has described, in general terms, hardwareand cabling. This section introduces the idea behind what the online curriculum calls networkingdevices and the role they play in creating networks. In particular, this section shows an exampleof a simple networking device called a LAN hub.
  18. 18. 18 Networking Basics CCNA 1 Companion Guide Companies create their LANs by connecting a cable between each computer and some net- working device, oftentimes either a LAN hub or LAN switch. In turn, these devices connect to each other. Designed and engineered properly, each computer can use one cable to connect to only one networking device but still can communicate with many other computers. The term networking device refers to a class of computer hardware devices that is designed for the specific purpose of building networks. Many types of networking devices exist, including two that are discussed throughout all four courses of the Networking Academy CCNA curriculum: switches and routers. For example, LANs can use one type of network device called a LAN hub (or simply hub). (The term hub comes from the idea of a wheel, in which the hub is in the middle and several spokes radiate from it.) A LAN hub has a large number of jacks (oftentimes, RJ-45 sockets) that connect the LAN cables attached to various PCs. By connecting each PC to the hub with a single cable, the network has an electrical transmission medium between each PC and the hub. When the hub receives electrical signals, it simply repeats those signals out all the other interfaces. Figure 1-12 shows an example of the cabling topology. Figure 1-12 Small LAN with a Hub Network Device Fred Wilma 0200.1111.1111 0200.3333.3333 I got an electrical signal in port 1; repeat it out the other three ports! 1 Barney Betty 0200.2222.2222 0200.4444.4444 With a hub, the PCs need only a single NIC and a single cable that connects them to the hub. Figure 1-12 shows PC Fred transmitting data (shown as a dashed line), with the hub repeating the transmitted bits to each device connected to the hub. If another PC joins the network, the new PC simply needs a NIC with a cable that connects it to the hub; the existing devices require no changes. A hub is just one example of a networking device that can be used to reach the main objective of networks: to allow multiple computers to communicate. LAN switches are another type of networking device similar to a LAN hub. A switch does the same work as a hub, but more effi- ciently, so today, switches are used more often than hubs. Also, routers (as shown in Figures 1-2 and 1-3) perform an important role. Routers can connect to wide-area networks (WANs), which provide a transmission medium across long distances. Chapter 2 closely looks at these network- ing devices, and later chapters further detail hubs, switches, and routers.
  19. 19. Chapter 1: Introduction to Networking 19The behavior of a LAN using a hub also provides a good backdrop from which to cover a topicrelated to NICs—namely, the concept of an address for a NIC. The company that makes theNIC gives it a unique permanent address. The address is 48 bits long and is typically written inhexadecimal as 12 hexadecimal digits. (Each hexadecimal digit represents 4 bits.) When writ-ing down these addresses, Cisco Systems tends to put two periods into the address to make itmore readable. For example, in Figure 1-12, Fred has a NIC address of 020011111111, whichis listed as 0200.1111.1111 on a Cisco product. The NIC address has many names besides NICaddress. The most common name is Media Access Control (MAC) address.When sending data, Fred adds a prefix to the data, including the MAC address of the intendedrecipient (for example, Barney). After the other three PCs receive the data, they use the destina-tion MAC address to decide whether to process the data (Barney) or not (Wilma and Betty).(This is just one example of how NICs use their MAC addresses; you will learn about manyother uses for MAC addresses before the end of this course.)Packet You can view a simulation of the network operation in Figure 1-12 by usingTracer Packet Tracer. Download the sample Packet Tracer scenarios from your login at, and load scenario NA01-0112. For more information, refer to this book’s Introduction.Enterprise Networks and Home Internet AccessSo far, this chapter has introduced three of the four main components of a computer network:computer hardware (including NICs), cables, and network devices. Today, most every enter-prise has a network installed, with the devices using many of the concepts covered thus far.This section closely looks at enterprise networks and then examines a few of the technologiesused to access the Internet.Enterprise Network BasicsEnterprises vary in many ways. The network created and owned by the college, school, or train-ing company at which you are taking this course mostly likely has a network. In fact, the PCsin the classroom might be connected to that network. If so, the PCs are actually part of theenterprise network that the school uses. An enterprise network is nothing more than a networkcreated to support the activities of that enterprise, whether that activity is to make money, gov-ern, or educate.
  20. 20. 20 Networking Basics CCNA 1 Companion Guide Enterprise networks can be large or small. Regardless of size, enterprise networks have many common requirements and features: ■ They might have several physical locations that are too far away to use a LAN. ■ The need to support many PCs for the people who use the enterprise network. ■ The need to connect servers to the network. Servers have information that is useful and important for the enterprise’s functions. ■ Typically, the computer-support engineers work near one or a few of the network’s main sites. ■ The need for Internet access for most or all of the PCs in the enterprise. To support such goals and requirements, an enterprise network might look like what’s shown in Figure 1-13. The figure shows an enterprise with four sites: one main site and three other sites. This enterprise could be a small business, local government, or even a school with four cam- puses. In Figure 1-13, Site 1 houses a server farm, which is a collection of servers located in the same location. The PCs throughout the enterprise network can access these servers through the enter- prise network. For example, a PC at Site 2 is shown communicating with one of the servers (as noted with the dark dashed line). Note that each PC is connected to a networking device called a LAN switch; the switch allows local communications over a LAN. (Switches achieve the same goal as hubs, but more efficiently, as covered briefly in Chapter 2 and in more depth in Chapter 7, “Ethernet Technologies,” and Chapter 8, “Ethernet Switching.”) The routers (hock- ey-puck icons) connect to the LAN and the WAN, forwarding traffic to and from the WAN con- nection. (WAN connections are often represented by a crooked line, sometimes called a light- ning bolt.) Only the main site has a connection to the Internet through ISP1. Although only Site 1 has the connection, the PCs throughout the enterprise use the enterprise network to reach router R6, which then forwards traffic into the Internet through ISP1. This design allows the users inside the enterprise to access the Internet, as well as users inside the Internet to access devices inside the enterprise. This enterprise-network design takes care of Internet connectivity for the entire enterprise. The next section covers the basics of Internet connectivity from the home.
  21. 21. Chapter 1: Introduction to Networking 21Figure 1-13 Typical Enterprise Network Server 1 Site 1 Server 2 SW6 SW1 R6 ISP1 Server 4 Server 3 R1 To the rest of the Internet R2 R3 R4 SW2 SW3 SW4 Site 2 Site 3 Site 4 Legend: LAN Cable WAN Connection ExamplesAccessing an ISP Through a Phone Line and an Analog ModemAs previously mentioned, many ISPs exist, with most vying for a share of the home Internetaccess market. As shown in Figure 1-2, to use the Internet from home, a PC must somehowconnect to an ISP. These ISPs try to get your attention through marketing and advertising andwant you to sign up with them to gain access to the Internet.To access the Internet, a home PC needs to use some transmission medium. With LANs, thetransmission medium is a cable that is relatively short, typically less than 100 meters in length.However, the distance between your house and the offices where the ISP keeps its networkingdevices might be long. Rather than install an expensive miles-long cable between your houseand an ISP, only to have you change to a new ISP in two months, ISPs use media (plural ofmedium) that are already installed in your house—namely, a phone line or a cable TV cable.This section describes how telephone lines work with analog modems. You learn more abouthow networks use cable TV lines in the section “Accessing an ISP Through a Cable TV Cableand a Cable Modem,” found later in this chapter.
  22. 22. 22 Networking Basics CCNA 1 Companion Guide When a phone call is placed, telephone companies essentially create an electrical circuit between two phones. When used to send and receive voice, the phones convert the sounds into an analog electrical signal by using a microphone built into the mouthpiece of the phone, send- ing that signal to the other phone. The receiving phone converts it back to sounds and plays it out a speaker built in to the phone’s earpiece. The analog electrical signal used by phones differs from digital electrical signals in that analog varies continuously, as shown in Figure 1-14. Figure 1-14 Analog Electrical Signal Voltage Amplitude Time Period 1 Second 3 Periods in 1 Second = 3 Hz Frequency Figure 1-14 shows the voltage level on the y axis over time, with time on the x axis. In short, the voltage continuously varies from some positive to negative voltage (in other words, alternat- ing current). Note that Figure 1-14 shows three complete cycles of the curve, beginning at the x axis, going up, down again, and back up to the x axis. The length of time it takes for one com- plete cycle to occur is called the period. The number of times that cycle occurs in 1 second is called the frequency. For example, the frequency shown in Figure 1-14 is 3 Hertz (Hz). (Hertz is a unit of measure that means “number of cycles per second.”) Finally, the maximum (posi- tive) and minimum (negative) voltage is called the amplitude. Phone companies originally used analog electrical signals for voice traffic because analog sig- nals look like voice waves as they travel through the air. Both electrical energy and sound waves vary in terms of frequency, amplitude, and so on. For example, the higher the frequency, the higher the pitch of the sound; the higher the amplitude, the louder the sound. An electrical signal sent by a phone is analogous to the sound waves; hence, the term analog describes this type of signal. Computers can use modems to send data to each other by sending analog signals, such as the signal shown in Figure 1-14. Two computers can essentially place a phone call to one another by using their modems. The modems then send binary 0s and 1s to each other by varying, or
  23. 23. Chapter 1: Introduction to Networking 23modulating, the analog signal. For example, the modems might use a higher-frequency signal tomean 1 and a lower frequency to mean 0. From the phone company’s perspective, however, itlooks just like any other phone call because all the modems do is send the same kinds of analogelectrical signals sent by a telephone.Modems might exist as a built-in feature on the computer motherboard or as a separate internalexpansion card. Modems might also be external to the PC, connecting to the PC via the PC’sserial port. Such a modem is aptly named an external modem.A Brief History of Remote AccessThis section briefly reviews the history of modems and Internet access, even before the adventof the Internet. The idea of remote access to the Internet from home did not become a main-stream offering until the early 1990s. Before then, however, devices like modems were used forother purposes. The following list outlines the major events in the history of remote access: 1960s—Modems were used by dumb computer terminals to access large computers called mainframes. 1970s—Bulletin Board Systems (BBSs) allowed original PCs to access a server, let you post a message for others, and look at messages posted by others. 1980s—File transfers between computers became popular, as well as rudimentary interac- tive graphics. 1990s—Internet access through a modem became popular, with modem speeds rapidly increasing to 56 Kbps. 2000s—High-speed Internet access became more affordable and more popular.The next two sections examine the high-speed Internet access methods that developed in thelate 1990s, which are now mainstream technologies.Accessing an ISP Through a Phone Line and a DSL ModemDigital subscriber line (DSL) defines a much higher-speed method of using the same homephone lines that modems use. DSL differs from how modems work in many ways, such as thefollowing: ■ DSL uses digital electrical signals, not analog. ■ Unlike modems, DSL allows a voice call to use the phone line simultaneously as DSL passes data. ■ To not interfere with a concurrent voice call on the same phone line, DSL uses frequencies outside the range typically used for voice. ■ DSL Internet service is “always on” in that no phone call or other effort must be made to access the Internet.
  24. 24. 24 Networking Basics CCNA 1 Companion Guide To use DSL, the home PC needs either an internal or external DSL modem or DSL router. Each of these devices understands, and uses DSL, but with slightly different features that are beyond the scope of the Networking Academy CCNA curriculum. Figure 1-15 shows a typical home installation with an external DSL router/modem. Figure 1-15 Basic Operation of Modems Andy’s PC IP Network Owned by ISP Ethernet Data Split to Digital Signals > ISP Router 4000 Hz Phone Line Analog Voice Split Telephone DTMF Tones, Network Andy’s Analog Voice, Analog Phone 0 – 4000 Hz Telephone Andy’s House Network Figure 1-15 shows how both the normal phone and the DSL router connect, through a typical telephone line with an RJ-11 connector, to a phone socket. The local telephone company then splits out the voice to the telephone network and splits out the data to give to an ISP. In effect, the PC now has an electrical path between itself and the router, with the ability to send bits to and from that router. In turn, this access provides the PC with Internet connectivity. Accessing an ISP Through a Cable TV Cable and a Cable Modem Cable TV companies provide Internet access that, from a nontechnical perspective, is similar to DSL. The PC needs either an internal or external cable modem or a cable router. The cable modem/router connects to the CATV cable instead of the phone line. The cable modem sends and receives data to and from a router inside the cable TV company; at the same time, the CATV cable is still available for its primary purpose: TV. In other words, you can watch TV and switch channels all you want, while someone else in the house uses the Internet over the same CATV cable. Like DSL, the service is always on and doesn’t require the user to do anything before beginning to use the Internet. Similar to DSL, it is fast, with download speeds well over 1 Mbps.
  25. 25. Chapter 1: Introduction to Networking 25TCP/IP Protocol Suite and TCP/IP SoftwareThe one network component that has not yet been covered in this chapter is software. Softwareprovides the motivation and the reasons why a computer tries to communicate in the first place.You might build a network with computer hardware, NICs, modems, cables, and networkingdevices, but if no software exists, the computers do not attempt to communicate. Software providesthat logic and that motivation for a computer to communicate.You might have used computer software that, in turn, caused the computer to use the network.If you have ever opened a web browser to look at web pages or surf the web, you have usedcomputer software that drives traffic across the network. In fact, because web browsers are socommonly used today, this section uses web browsers as examples.Networking Standards, Protocols, and the TCP/IP Protocol SuiteFor computer communications to be useful, the communication must follow a set of rules.Networking rules are formally defined by many different networking standards and networkingprotocols. Individually, a single networking standard or networking protocol defines the rulesfor a small part of what a network does. For example, an encoding scheme used by an EthernetNIC would be a single standard. This section looks at a few networking protocols as examples,namely Hypertext Transfer Protocol (HTTP) and Internet Protocol (IP).Computers and network devices implement protocols mainly through computer software. For Noteexample, when you download a web page, the web browser uses HTTP, which the web-browser A computer OS is softwaresoftware implements. To deliver the data to and from the web server, the PC might use other that controls the computerprotocols, such as the aforementioned IP. Today, most computer OSs implement IP, so IP is hardware, providing aalready built in to the OS and available for use. human interface to the computer. Windows XPFor a network to work, all network components must use the same set of standards and proto- and Linux are two exam-cols. Many options exist for standards and protocols to do the same (or similar) functions, so to ples of PC make sense of all that, a concept called a protocol suite or networking model was created.A protocol suite is a set of protocols through which, when a computer or networking deviceimplements many of the protocols in the suite, the computers can communicate easily andeffectively.The Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite defines and col-lects a large set of networking standards and protocols that are used on most computers today.The concepts and protocols covered in this section, including HTTP and IP, are part of theTCP/IP protocol suite. Chapter 9, “TCP/IP Protocol Suite and IP Addressing,” covers theTCP/IP protocol suite (also called the TCP/IP networking model) in depth.
  26. 26. 26 Networking Basics CCNA 1 Companion Guide Using HTTP to Download a Web Page This section covers what might be a familiar topic: the use of a web browser. What is probably new is how web-browser software implements one of the many protocols inside the TCP/IP protocol suite (specifically, the HTTP protocol) to get the contents of a website from a web server. In case you’re less familiar with web browsers and web servers, the next section briefly introduces these concepts. Web Browsers and Web Servers Web-browser software shows information in a window of a PC’s video display. That informa- tion might be simple text, graphics, video, or animation. The browser can also play audio. Today, the most popular web-browser software comes from Microsoft, called Internet Explorer (IE). Netscape and Mozilla Firefox are two other popular web browsers. A web server is software that distributes information from the web server to web browsers. For example, Cisco Systems has a website ( that lists tons of information about its products and services. Cisco creates the website by placing the web pages on a server, installing web server software on that server, and telling the web server software to supply the web pages to any browsers that request the web pages. Note that many people use the terms website and web page to refer to web-based content. The term website refers to a bunch of related content. For example, in the case of Cisco, if you spend time clicking different links in the browser after starting at, everything you look at is part of the Cisco website. At any one point in time, however, you look at an indi- vidual web page. Web browsers can display a large variety of content, but, in some cases, they also need help. For example, browsers can easily display text and graphics. For some functions, however, the browser relies on other software, with the browser placing the other software’s display window inside its window. For example, a browser might not directly support the ability to show a video, but it might instead use software that plays a video. The Microsoft IE browser, for exam- ple, might use the Microsoft Windows Media Player (WMP) to show a video. Downloading a Web Page Computers store long-term data on disk drives in an object called a file. Computers, in their most basic form, work with binary digits, which people commonly abbreviate to bits. However, when describing computers, it is cumbersome to discuss and work with every little bit. So, computers combine 8 bits into a byte for a small amount of added convenience. The next step is to combine a set of bytes into a file. Computer files hold a set of related information. For exam- ple, a single computer file might hold a homework assignment you typed, a graphical image, a song that an MP3 player can play, a video, or any other single entity that computer software might want to manipulate or use.
  27. 27. Chapter 1: Introduction to Networking 27When a web developer creates a web page, the result is a set of files stored on the web server.One or more files might hold the web page’s text. Other files hold graphics (typically, one fileper graphical image). Other files might hold audio that plays when you load the web page.When a browser loads a new web page, the following sequence of events occurs: 1. The browser asks the server to send one file that has both instructions and displayable content. 2. The browser displays the contents of the file. 3. The browser also looks at the instructions inside the file, which might tell it to get more files from the web server. 4. The browser asks the server for the additional files. 5. The browser displays the additional content, as well as looking for additional instructions to download other files. 6. The browser continues to look for instructions to download other files that are part of the web page until all files are downloaded and displayed.To actually request the files and to cause the files to be transferred from the server to the webbrowser, the browser and server both use HTTP. Figure 1-16 depicts the flow and logic of howthe web browser transfers files.Figure 1-16 HTTP Transfers Three Files The home page file was home.html. It told me to get logo1.gif and ad1.gif from directory /graphics. HTTP GET (default) Web HTTP OK data: home.html Browser Software HTTP GET /graphics/logo1.gif HTTP OK data: logo1.gif HTTP GET /graphics/ad1.gif HTTP OK data: ad1.gifHTTP uses the concept of a get request. The HTTP get request identifies the file that thebrowser needs from the server. The server obliges and sends the file. In this case, the first file,called home.html, holds instructions telling the browser to ask for two more files.
  28. 28. 28 Networking Basics CCNA 1 Companion GuideNote Browser Plug-InsThe term HTTP is derived Browsers know how to display the information in HTML and other types of files. However,from the first type of file rather than having to understand how to display any and every type of content, oftentimes thesupported by a web brows-er: a file with Hypertext browser uses another program to display some types of content. For example, browsers oftenMarkup Language (HTML) use a program called Flash Player ( to display animations like what’stext and instructions. shown in the online curriculum. For video or audio, browsers often use products such asOriginal web browsersneeded to download HTML QuickTime (, RealPlayer (, or WMPfiles exclusively and, to do (, they needed a protocolto transfer the HTML files. When a browser needs to use one of these programs, it typically displays the related informa-So, HTTP is derived from tion inside the browser window on the PC display screen. In effect, this additional program isthe idea of a protocol to “plugged in” to the browser window. As a result, many people refer to these programs as plug-ins.transport hypertext. Lab 1.1.8 Web Browser Basics In this lab, you learn how to use a web browser to access Internet sites, become famil-Note iar with the concept of a Universal Resource Locator (URL), and use a search engineFlash Player from to locate information on the Internet. You access selected websites to learn the defini-Macromedia is requiredbefore you can see all tions of networking terms and use hyperlinks to jump from a current website to otherthe content of the course. It canbe downloaded TCP/IP Networking Model As previously mentioned, TCP/IP consists of a large number of protocols. In fact, the name TCP/IP refers to two of the more popular protocols inside TCP/IP: namely, Transmission Control Protocol (TCP) and Internet Protocol (IP). Because TCP/IP contains such a large vol- ume of protocols, it is useful to think about the TCP/IP protocol suite by grouping its member protocols into categories called layers. Figure 1-17 shows the TCP/IP networking model and its component layers. Figure 1-17 TCP/IP Networking Model and Protocols TCP/IP Model TCP/IP Protocols HTTP, SMTP, Application POP3 Transport TCP, UDP Internet IP Network Ethernet, Interface Frame Relay PPP
  29. 29. Chapter 1: Introduction to Networking 29The TCP/IP networking model, like other networking models, shows several layers on top ofeach other. Each layer implies a general category of service or function that the protocols atthat layer perform. For example, the application layer provides services to applications. Webbrowsers are concerned with displaying information on the screen, but to do so, they ask forhelp from a TCP/IP application layer protocol (namely, HTTP).Chapter 2 covers the details about networking models in general, referencing another popularnetworking model called the OSI reference model. Chapter 9 covers the details about theTCP/IP networking model. This chapter covers the most basic features of IP, TCP, IP routing, and IP subnetting; however, the online curriculum does not cover these topics until Modules 9 and 10. The book has included this additional coverage of these very practical parts of networking to balance the highly-theoretical coverage of the early chapters of this book. Also, many details in this section relate to the trou- bleshooting tools introduced in module 1 of the course. If you prefer to skip topics that are not mentioned in the online curriculum, skip forward to the section “Troubleshooting Basics for IP.”TCP/IP Transport Layer and the TCP ProtocolThe TCP/IP transport layer is a group of protocols that provide services to application layerprotocols. In fact, each layer of any networking model provides services to the layer just aboveit. This section describes one example of how a protocol at one layer provides a service to theadjacent higher layer by using the TCP and HTTP protocols in the example.TCP happens to provide an important and popular service to many application protocols: theservice of guaranteed delivery of data. Using HTTP (application layer) and TCP (transportlayer, one layer below the application layer) as examples, consider the difference in logic usedby the software on a web server: ■ HTTP—I need to transfer files to another computer’s web browser, but I do not have any way to recover data in case it gets lost. ■ TCP—I have the capability to monitor transmitted data to determine whether the data arrived. If it gets lost, I can resend the data, which guarantees that all the data eventually arrives safely.As you can see, the two protocols seem like they were made for each other. In fact, TCP pre-dated HTTP by quite a few years, so when HTTP was created, the people who created HTTPsimply decided to use TCP to perform guaranteed delivery of the data through TCP’s error-recovery process. Figure 1-18 shows how the actual error-recovery process works.
  30. 30. 30 Networking Basics CCNA 1 Companion Guide Figure 1-18 Example of TCP Error Recovery PC1 Web Server #1 First Part of file… #2 X Second Part of file… #3 Third Part of file… Please resend #2 #2 Second Part… Again The TCP software, built in to the OS on the server, marks each packet with a sequence number, as Figure 1-18 shows. Because the second packet was lost, the PC on the left receives only the packets numbered 1 and 3. From that, the TCP software on the PC on the left—again, software built in to the OS on the PC—can surmise that packet 2 was lost somewhere along the way. To cause the server to resend the lost packet, PC1’s TCP software sends a request to the server to resend packet 2, which it does. The next section introduces IP, which provides the service of routing the packets from one computer to another. TCP/IP Internet Layer and the Internet Protocol The single most important protocol in the TCP/IP networking model is IP. It exists as part of the internet layer in the TCP/IP networking model. IP defines many things, but two features stand out by far as the most important. A large percentage of your work, thought, and learning in the Networking Academy CCNA classes somehow revolve around IP and its main two features: ■ Routing ■ Logical addressing These features are linked in many ways, so they are best understood together. Although this chapter provides an introduction to networking, including networking software that implements TCP/IP, learning the basics of IP now helps provide some insight into where you will go in this class.
  31. 31. Chapter 1: Introduction to Networking 31IP Addressing and RoutingNetworks that use TCP/IP, which includes almost every network today, assign an IP address toeach network interface. The IP address uniquely identifies that interface inside the network.After each network interface has a unique IP address, data can be sent from device to device byusing these IP addresses, delivering the data to the one device that uses a particular IP address.The preceding paragraph, although accurate, is relatively generic, so some concrete examplescan help. IP addresses are 32-bit binary numbers. To make things easier on us humans, IPaddresses might be written in dotted-decimal form. Dotted decimal means that the 32-bit num- Tipber is represented by four decimal numbers separated by periods (dots). Each decimal number The four decimal valuesrepresents 8 bits. For example, the following line shows the same IP address in both binary inside an IP address eachform and dotted-decimal form: represent 8 bits. Most peo- ple use the term octet to 00001010000000010000001000000011 refer to a single one of the four numbers inside an IPCertainly, it is much easier to keep track of the dotted-decimal version of the IP address than address. The decimal val-the binary version of the same IP address. In this case, the decimal 10 represents the first 8 bits, ues must be between 0 andthe 1 represents the next 8 bits, and so on. For example, the 8-bit binary equivalent of decimal 255 and inclusive. Chapters 9 and 10 cover additional10 is 00001010, which begins the 32-bit IP address just shown. restrictions.Now, consider the simple LAN shown in Figure 1-19. It has two PCs, each with an IP address.Figure 1-19 Single LAN with Two PCs and One Router R1 SW2 PC21 PC22IP defines that each network interface that uses IP must have an IP address. In Figure 1-19,each PC has a single Ethernet LAN NIC. A NIC provides an interface to the network so thePCs can assign an IP address to the NIC. In fact, you can easily see the IP address assigned to aNIC on most any computer, if you know the details. On Microsoft Windows XP, for example,you can use the ipconfig command from a command prompt, as shown in Figure 1-20. Figure1-20 shows the ipconfig command output from PC21 in Figure 1-19.
  32. 32. 32 Networking Basics CCNA 1 Companion Guide Figure 1-20 Sample ipconfig Output Packet The ipconfig command shown in Figure 1-20 can be performed using the Packet Tracer Tracer tool in Real-time mode. Feel free to experiment with the NA01-0112 con- figuration previously used in this chapter, and look at the IP addresses on Fred, Barney, Wilma, and Betty. Using these IP addresses, the PCs can send each other packets of data. The packet includes end-user data and headers added by some other protocols, including the IP header added by the IP software on a computer. The IP header includes a source IP address and a destination IP address. Lab 1.1.6 PC Network TCP/IP Configuration In this lab, you learn the methods of discovering your computer’s network connection, hostname, MAC (Layer 2) address, and network (Layer 3) address.NoteIP addresses are considered IP Address Organization: Subnetsto be logical addresses. Theterm logical is not meant to Figure 1-21 adds a router and two switches to the network shown in Figure 1-19. In this net-imply that other addresses work, the router is connected to three LANs, so it has three network interfaces, one connectedare “illogical,” but rather to each LAN. Therefore, the router has an IP address for each interface because IP addressesphysical. For example, a uniquely identify interfaces used to connect to a network.MAC address is perma-nently associated with a The IP addresses in Figure 1-21 must conform to certain rules. In this example, all IP addressessingle physical NIC, so aMAC address is a physical on the same LAN must use the same first three decimal numbers in their respective IP address-address. An IP address can es. Figure 1-22 shows how the numbers are assigned to any PC, soIP addresses are not physi-cal.
  33. 33. Chapter 1: Introduction to Networking 33Figure 1-21 Network with Three LANs Connected by One Router PC11 PC12 SW1 PC31 Fa0/0 Fa0/2 R1 SW3 Fa0/1 PC32 SW2 PC21 PC22Figure 1-22 Grouping Effect of IP Addresses All Addresses that begin 172.16.1 Fa0/0 All Addresses that begin 172.16.3 R1 Fa0/2 Fa0/1 All Addresses that begin 172.16.2
  34. 34. 34 Networking Basics CCNA 1 Companion Guide Figure 1-22 shows three separate IP subnets. A subnet is a group of IP addresses that share a common value for the beginning parts of the IP addresses in the subnet. Also, the members of a subnet must not be separated from each other by any routers. The three subnets in Figure 1-22 are loosely described as follows: ■ All IP addresses beginning with 172.16.1 ■ All IP addresses beginning with 172.16.2 ■ All IP addresses beginning with 172.16.3 IP Routing with IP Routers The rules about the IP addresses shown in Figure 1-22 allow for easy routing. The router keeps a table—called a routing table—that essentially lists all the groups of IP addresses that it can somehow reach and the interface out which it needs to send packets to reach those groups. For example, for Figure 1-22, router R1’s routing table contains the equivalent of what’s shown in Table 1-1. Table 1-1 Pseudo-Routing Table on Router R1 in Figure 1-22Note To Forward Packets Sent to Send the Packets Out This InterfaceFa is short for Fast Addresses That Begin With…Ethernet, which is a type ofEthernet LAN that sends 172.16.1 Fa0/0bits at 100 Mbps. 172.16.2 Fa0/1 172.16.3 Fa0/2 Armed with the information in Table 1-1, the router can receive data packets sent by any of the PCs in the network and make the correct decision about where to forward the packets. Troubleshooting Basics for IPNote Now that you know the very basics of IP addresses and routing, the rest of this section coversFor those of you skipping troubleshooting. Frankly, it is a bit ambitious to think about troubleshooting before coveringthe extra coverage of TCP, more of the course. However, you can do some basic things that are both interesting and funIP, IP subnetting, and IP from any network-connected PC.routing, begin readingagain at this heading. The online curriculum uses a how-to list, similar to the following one, that suggests general steps in how to approach problems when you troubleshoot: How To Step 1 Define the problem. Step 2 Gather the facts. Step 3 Identify possible solutions by doing the following:
  35. 35. Chapter 1: Introduction to Networking 35 1 Analyze the facts compared to the expected behavior of the network. 2 Determine possible root causes of the unexpected behavior. 3 Determine a set of actions that might solve the problem. Step 4 Create an action plan to implement the solution you chose in Step 3. Step 5 Implement the plan. Step 6 Observe the results. Step 7 Document the details. Step 8 Introduce problems and troubleshoot.One of the most valuable tools when troubleshooting any network is to have ready knowledgeor references that describe how the network is supposed to work. The computers and networkdevices that comprise a network do attempt to follow the protocols, and these protocols define aseries of steps that must be taken under certain conditions. If you know the steps that shouldoccur, you can try to find the first step that is not working correctly and then look for reasonswhy that step failed.This section describes some of the basics of IP addressing and routing, to complete the pictureof what should happen, along with some tools that are useful for troubleshooting.Name Resolution Using a Domain Name System ServerAlthough a basic understanding of IP addressing is useful at this point in the class, most com-puter users never even think about IP addresses. However, they do know the names of thingsthat can be translated into IP addresses. For example, you might see an advertisement on TVor in a magazine that mentions a website. The ad might list something such as You might open a web browser and plug that into the field called Address at thetop of the browser, and the website suddenly appears. Or you might already be on some web-site and click something on the screen, and another website appears. When you do that, thebrowser’s Address field changes because your clicking action causes the web browser to jumpto another web address.The correct term for the text placed into the Address field of a web browser is UniversalResource Locator (URL). Many people simply call this a web address. The URL itself hassome structure. For example, the following URL represents the web page from which you canlook at all the Cisco Systems documentation: closely examining that URL, you can separate the URL into its components. First, thestuff before the double slash simply identifies the protocol to use, which, in this case, is the