The first widely used internet protocol suite was TCP/IP. Its name combines the Internet Protocol (IP), which addresses and routes packets, with the Transmission Control Protocol (TCP), which provides reliable, ordered delivery between applications.
TCP/IP grew from research funded by DARPA in the 1970s. Vint Cerf and Robert Kahn helped develop the internetworking design, which allowed different kinds of networks to communicate. On January 1, 1983, ARPANET officially switched from its earlier Network Control Program to TCP/IP, a milestone often called “flag day.”
TCP/IP is not one single protocol but a family organized into functional layers. IP handles internetwork delivery, TCP handles reliable transport, and application protocols such as HTTP, DNS, and SMTP operate above them. UDP is another important transport protocol in the suite.
A common mix-up is the OSI model. OSI is a seven-layer reference model, while TCP/IP is the practical protocol suite that became dominant. TCP and IP are also separate protocols, even though they are commonly named together.