And for completeness, each IP address has it's own range of ports. i.e. if you have a server listening on 127.0.0.1:3306, you could still open a listening socket on 192.168.1.1:3306 (assuming the box has a network interface with that IP).

Commonly, applications specify 0.0.0.0 as the IP address to listen on, which means "the IP addresses of all interfaces, which are currently up".

In an environment where network interfaces go and up and down, this can be a real issue, since the number of interfaces your 0.0.0.0 server will listen on depends when it is started.