January 1, 1983, is often called the internet’s “flag day”: the date on which the ARPANET stopped using its original Network Control Program and switched entirely to the TCP/IP protocol suite. A flag day is a coordinated transition where everyone changes over at the same moment rather than gradually, and this one marks the point where the ARPANET began running the same core protocols that the global internet still uses.
The original ARPANET host-to-host protocol was defined in documents like RFC 33, “New HOST-HOST Protocol,” written by Steve Crocker with Steve Carr and Vint Cerf and dated February 1970. It introduced the Network Control Program (NCP), a 40-bit socket naming scheme, and connection setup and teardown commands. NCP served the ARPANET through the 1970s, but it assumed a single reliable network rather than an interconnection of many different networks, which is what the new internetworking research aimed to support.
The transition was planned in RFC 801, “NCP/TCP Transition Plan,” written by Jon Postel of ISI in November 1981. After the Department of Defense adopted IP and TCP as standard protocols, the plan set a schedule: hosts were to begin implementing IP/TCP through 1982, with the full switchover targeted for January 1, 1983. By that date NCP was to be removed from service, all services were to run over TCP, and temporary relay hosts that bridged the two worlds during the changeover would be shut down. To smooth the transition, many hosts ran both protocols at once.
The cutover was not painless. Sites that had not finished their TCP/IP implementations were effectively disconnected until they caught up, and the changeover required substantial coordination across the dozens of hosts and host operating systems then on the network. But the deadline forced the issue, and afterward the ARPANET ran a clean, uniform internet protocol stack.
Flag Day matters because it converted TCP/IP from a research protocol into the operational foundation of a working network. The architecture that Vint Cerf and Bob Kahn had designed, and that Postel and others had refined through years of RFCs, became the live protocol base on January 1, 1983. Every later expansion of the internet, from NSFNET to the commercial internet of the 1990s, built on the protocol suite that this single coordinated transition put into production.