Host addresses (127.0.0.1, 0.0.0.0)

Also written: 127.0.0.1, 0.0.0.0, localhost

`127.0.0.1` (named `localhost`) always means "this machine", so a service listening only there can't be reached from outside. `0.0.0.0` means "every address this machine has", so anyone who can reach the server can reach that service.