127.0.0.0/8 - Loopback
127.0.0.0/8 was allocated on September of 1981.

Addresses in 127.0.0.0/8 may NOT be used as the source address of an IP datagram that transits two devices.

Addresses in 127.0.0.0/8 may NOT be used as the destination address of an IP datagram that transits two devices.

Addresses in 127.0.0.0/8 are NOT forwardable.
A router may NOT forward an IP datagram to Addresses in 127.0.0.0/8 between external interfaces

Addresses in 127.0.0.0/8 are NOT globally reachable
An IP datagram may NOT be forwarded to Addresses in 127.0.0.0/8 beyond a specified administative domain.

The RFC that created 127.0.0.0/8 requires all compliant IP implementations to behave in a special way when processing packets either to or from addresses contained by 127.0.0.0/8

127.0.0.0/8 is the Internet host loopback address block. A datagram sent by a higher-level protocol to an addreess anywhere within this block loops back inside the host. This is ordinarily implemented using only 127.0.0.1 for loopback. Addresses within the entire 127.0.0.0/8 block do not legitimately appear on any network anywhere.

[ RFC1122, ^ ] , Section 3.2.1.3

However, several protocols have been granted exceptions to these rules.

For examples of exceptions see [ RFC8029, ^ ] , [ RFC5884, ^ ]