January 1, 1983

ARPANET

ARPANET makes TCP/IP its standard in a flag-day protocol cutover

Participants received buttons reading “I survived the TCP/IP transition.” The supposedly instantaneous flag day had taken years to plan.

1 January 1983: convert or rely on temporary relays

On 1 January 1983, ARPANET changed its standard host protocol suite from the older Network Control Protocol to TCP/IP. The date was the compulsory target in Jon Postel's RFC 801 transition plan, whose stated goal was a complete switch from NCP to IP/TCP by that day.

The cutover was organized as a “flag-day” transition: hosts needed to convert together, or rely temporarily on improvised or relay mechanisms to communicate across the protocol divide.

Why a deadline was necessary

ARPANET's NCP protocol was designed for communication inside one network. DARPA needed a common suite capable of carrying traffic across ARPANET, packet-radio, satellite and other independently operated networks, so it imposed a coordinated conversion deadline.

Standardizing on TCP/IP let ARPANET operate as part of an internetwork made from different packet-switched networks, rather than as an isolated network using an ARPANET-only host protocol.

Neither instant nor an invention

TCP/IP was not invented on New Year's Day. Implementations and experiments had been developed and tested for years beforehand, and the Defense standard had been adopted about three years earlier, in 1980. Each ARPANET institution had to install the new protocols on its own machines; there was no central software switch controlling every host.

“Officially completed” is likewise too literal for the date: converted hosts could join the growing internetwork while lagging hosts faced restricted or temporary relay arrangements, and an IEEE historical account says every host was running TCP/IP by June 1983.

Sources

Researched 23 Aug 2026 7 sources 3 audit passes, 1 issue open

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

What the sources leave uncertain

The date is secure as the mandatory cutover and standardization date, but “officially completed” is too literal: some host conversion work persisted, and an IEEE historical account says every host was running TCP/IP by June 1983.
Calling this the Internet’s “birthday” is an interpretive convention. Internetworking experiments existed earlier, and the Internet continued evolving afterward.