ARPANET officially switched to the TCP/IP protocol suite in 1983.
On January 1, 1983, often called Flag Day, ARPANET moved from its earlier Network Control Protocol to TCP/IP. The change gave the network a more flexible way to connect separate networks, which is why TCP/IP is closely associated with the development of the modern Internet.
TCP/IP is not one single protocol. Transmission Control Protocol manages reliable delivery and ordering of data, while Internet Protocol handles addressing and routing packets between networks. Other protocols operate alongside them for functions such as naming, web traffic, and email.
The 1983 transition did not instantly create today’s Internet, and other networks already existed. However, adopting a common internetworking protocol made it easier for independent networks and institutions to communicate as a larger system.