The Day the Internet Switched On
On this day · 1 January 1983
On a single coordinated "flag day", ARPANET threw out its old protocol and adopted TCP/IP, the language the whole internet still speaks.
On January 1, 1983, every machine on the ARPANET had to do the same thing at once: abandon the old Network Control Protocol (NCP) and switch to TCP/IP, the protocol suite devised by Vinton Cerf and Robert Kahn. Engineers called it a “flag day” because there was no gradual rollout. A host either spoke the new language by midnight or fell silent.
NCP could only talk within ARPANET itself. TCP/IP was built for internetworking, letting separate, independent networks interconnect and pass packets as one. That single design choice is why the system that followed earned the name the internet.
“One of the more interesting challenges was the transition of the ARPANET host protocol from NCP to TCP/IP as of January 1, 1983.”
The cutover had been planned for years, yet it still rattled administrators across universities, labs, and military sites. It went surprisingly smoothly, and the protocols chosen that day remain the foundation of essentially all global networking more than four decades later.
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