CIDR Calculator Online - How to Subnet

CIDR Calculator

How To Use

  • Input IP address. Incomplete IP addresses will be automatically finished with zeros if necessary;
  • Invalid IP addresses will appear read and will show no calculation results;
  • Select CIDR netmask or related parameter - max networks or addresses;
  • CIDR calculator will calculate CIDR notation, wildcard mask, maximum subnets and addresses, and CIDR address range;
  • Calculator can be used to plan set up of large or medium networks or for the studying.


This tool is used to calculate different network parameters (IP route, broadcast, address range etc.) when classless inter-domain routing method of allocating IP addresses and routing IP packets is being used. Classful method was replaced by CIDR back in 1993 and the motivation was a necessity to reduce the growth of routing tables and to use IPv4 address pool more effectively. For classful IP address calculations, please use Subnet calculator.


Classless inter-domain routing is based on VLSM (variable length subnet mask). It allows networks to be divided into different size and type subnets while not wasting any of the IP addresses. For example, network can be divided into two /26 and one /25 subnets -, and Reversed process of splitting networks into variable length subnet mask subnets is called supernetting. To calculate supernets please use Supernet calculator

More detailed information about VLSM can be found in RFC 950.


The Mask (routing prefix) shows what part of an IP address is network part and which is a host part. CIDR mask in IP version 4 is 32 bits long and is usually represented using dotted decimal form. Valid CIDR masks binary form starts with single or multiple ones (ones part) followed by stream of zeros (zeros part). After first zero there can only be ones. For example, 11111111.11111111.11110000.00000000 (

Maximum Subnets

Maximum number of subnets available for the specified CIDR mask. It calculated by: 232-routing_prefix

Maximum Addresses

Maximum number of addresses available for the specified CIDR mask.It calculated by: 232-routing_prefix - 2. Exception are networks with "/32" masks: 232-routing_prefix.

CIDR Notation

CIDR notation is a format of IP address and the routing prefix. This format consists of network ID calculated from IP address and CIDR mask and from CIDR mask itself as a routing prefix (for example, or CIDR notation is also used for IPv6 routing.

Wildcard Mask

Wildcard mask is inverted CIDR mask and it is calculated by performing a bitwise NOT on CIDR mask:

NOT 11111111.11111111.11111111.00000000 (
  = 00000000.11111111.11111111.11111111 (

Address Range

Range of addresses that selected IP address and subnet mask covers. The first is calculated by add 1 to network ID and last is calculated by subtracting 1 from broadcast.