Overview#Loopback is defined in RFC 5735 as a Global and Other Specialized Address Blocks stating:
127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher-level protocol to an address anywhere within this block loops back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback. As described in RFC 1122, Section 22.214.171.124, addresses within the entire 127.0.0.0/8 block do not legitimately appear on any network anywhere.