Minecraft wiki entry for Setting_up_a_server, associated text: When you disable the router/modem firewall and test your public (non RFC 1918) IP address on a port forwarding-checking website, the website will hit your router/modem, and your router/modem will respond, yes you can see me. This is another reason why disabling your firewall is bad; you incorrectly believe that people outside your network can connect to your Minecraft Server on your computer, when really, they're trying to connect to the router/modem itself.