The loopback IP range starts at 127.0.0.0 and reaches to 127.255.255.255. The IP stack automatically short-circuits any packets that are sent to a loopback IP, and never puts them onto a real network. Instead, the packets get routed to the
localhost, the
network interface that the packets were sent from.
The loopback IP exists independently of any of the interfaces
contactable IP addresses, and is always available, even if the interface is down (this is due to the fact that the packets sent to a loopback IP are intercepted in the software
TCP/IP stack, and never makes it to the hardware interface).