loopback (127.0.0.1, lo)

The interface every Linux machine carries, lo, holding the address 127.0.0.1: a packet sent there is delivered by the kernel back to the same machine without ever touching the network card. It exists so processes on one host can talk over the same sockets they would use across a network — an application reaches its database on 127.0.0.1:5432 with the code it would use for a remote one. Two consequences, pulling opposite ways. Loopback answers when everything else is broken: interface down, cable out, provider offline. And a curl http://127.0.0.1/ run on the server proves the process is listening and nothing else — not the firewall, not the default route, not the provider's address translation. "It works on the server, so the network is fine" is that test being over-read.