ARPANET adopted the TCP/IP protocol suite on January 1, 1983, creating a foundation for the modern Internet.
The transition, often called the “flag day,” replaced ARPANET’s earlier Network Control Program with the Transmission Control Protocol and Internet Protocol. IP handled addressing and forwarding packets between networks, while TCP provided reliable, ordered delivery for applications that needed it.
TCP/IP was important because it allowed separate networks with different underlying technologies to communicate through a common internetworking architecture. This design supported the growth from a research network into a network of networks.
TCP and IP are not the only protocols in the suite. UDP provides a simpler connectionless transport, while DNS, SMTP, HTTP, and many other protocols operate above or alongside the core layers. The phrase “TCP/IP” therefore refers to a family of interoperating protocols rather than one single protocol.