RFC 791: Internet Protocol Alive

RFC 791 (September 1981) specified IPv4: the 32-bit address space, header format, fragmentation, and the design choice that made it all work — IP itself is connectionless and unreliable; reliability lives at the edges. Postel edited the specification, as he did for nearly every foundational RFC.

The design's genius is its modesty: the network promises almost nothing, so anything can run over it. IPv4's successor took thirty years to draft and is still deploying; Postel's protocol, meanwhile, has carried the internet since 1983.