The loopback device is a special, virtual network interface meant to be used mainly for diagnostics and troubleshooting, along for enabling connecting to servers running on the local machine.
It works fundamentally by returning the packets sent to it (loopback!), in such a way that services can talk with themselves. This device implements no physical layer of the network stack, packets are passed without that layer.
Loopback will always have the static IP addresses 127.0.0.1/8 and
::1/128. These addresses can be changed in runtime.
config_netinter.