What is ipv4.cat?

ipv4.cat is a server that simply replies with a remote address on all TCP ports.

How is this useful?

This provides a way of determining your public IP address from command line with the least amount of typing possible, without requiring the installation of any software. Similar to portquiz.net, it also provides a way of checking for outbound firewall blocks. ipv4.cat differs from services like ifconfig.me in that it returns a raw TCP response and therefore doesn't require a HTTP client.

Example uses

ftp is recommended, as it is short, the most universal method across operating systems and is included in Windows by default (unlike telnet).

ftp ipv4.cat
telnet ipv4.cat
ncat ipv4.cat
curl ipv4.cat