What happened January 1983 AD

ARPANET became the technical core of what would become the Internet, and a primary tool in developing the technologies used. The early ARPANET used the Network Control Program (NCP, sometimes Network Control Protocol) rather than TCP/IP. On January 1, 1983, known as flag day, NCP on the ARPANET was replaced by the more flexible and powerful family of TCP/IP protocols, marking the start of the modern Internet.

The software was redesigned as a modular protocol stack, using full-duplex channels. Originally named IP/TCP it was installed in the ARPANET for production use in January 1983.