Foundations
Localhost
Also: 127.0.0.1, loopback
The address that always means this machine itself, so traffic to it never leaves the computer.
In more detail
The IPv4 loopback address is 127.0.0.1. A service that listens only on localhost can be reached by programs on the same machine but not by other computers, which makes it a simple way to keep a development server or database off the network.