ARPANET switched from NCP to TCP/IP in 1983.
The change was completed on January 1, 1983, a date often called “flag day” in Internet history. On that day, ARPANET permanently adopted the TCP/IP protocol suite, replacing the older Network Control Protocol (NCP) as its standard method for communicating between networked computers.
TCP/IP was more flexible than NCP because it was designed to connect multiple networks rather than merely operate one centrally managed network. TCP handled reliable delivery and the ordering of data, while IP handled addressing and routing packets across interconnected networks. This architecture helped turn ARPANET's networking principles into the foundation of the modern Internet.
The transition did not mean the entire Internet suddenly appeared in 1983, nor did it make ARPANET the first computer network. It was a crucial standardization milestone. Domain Name System development followed later in the 1980s, and the World Wide Web was not proposed until 1989.