Retronetworkingipv4rfclocalhost
Jon Postel Reserved All 16 Million 127 Addresses in 1981, Two Years Before Anyone Chose 127.0.0.1 for Loopback
RFC 790 marked network 127 reserved with no stated purpose; Berkeley Unix picked 127.0.0.1 in 1983, RFC 990 made 127 loopback in 1986, and a 2025 draft proposed giving most of it back.
A 2025 Internet-Draft by Seth Schoen, John Gilmore and Dave Täht, Unicast Use of the Formerly Special-Cased 127/8, went looking for the reason. It says network 127 was “reserved for a then-unspecified purpose by Jon Postel in 1981.” Postel’s habit was to “reserve the first and last network of each class,” and the authors add that “it does not appear that he had a specific plan for how to use 127/8.”
The loopback choice came from Berkeley. The draft credits “experimental Berkeley Unix releases by Bill Joy and Sam Leffler” and says “the choice of 127.0.0.1 as loopback address was made in 1983,” in the work that became 4.2BSD. The earliest code, it says, “used 254.0.0.0 and 127.0.0.0 as loopback addresses.”
The standards caught up three years later. RFC 990, from November 1986, says “the class A network number 127 is assigned the “loopback” function,” and “no datagram “sent” to a network 127 address should ever appear on any network anywhere.”
A block this size sitting mostly idle invites proposals like this one. The harder part is every stack, firewall rule and test suite that already assumes all of 127 loops back.