On what date did ARPANET officially switch to TCP/IP?

ARPANET officially switched from its Network Control Protocol to TCP/IP on January 1, 1983. The date became known as “Flag Day” because networked machines were expected to make the protocol change together.

Before 1983, ARPANET used NCP, an earlier host-to-host communication protocol. TCP/IP offered a more flexible architecture for connecting different kinds of networks, which was essential as internetworking expanded beyond one experimental packet-switched network. On Flag Day, TCP/IP was permanently activated across ARPANET, while NCP was retired for normal operation.

The switch did not create the Internet overnight, and TCP/IP had already been developed and tested before the deadline. The significance of January 1 was coordination: a large, established network moved to a common protocol suite that could link independent networks. The transition helped establish the technical foundation of today’s Internet.