ARPANET
also: Advanced Research Projects Agency Network
ARPANET was the earliest wide-area computer network, created by the U.S. Department of Defense's Advanced Research Projects Agency (ARPA) in 1969. It is the direct predecessor to the modern Internet.
ARPANET connected research institutions and universities across the United States, pioneering packet-switching technology and distributed network architecture. It demonstrated that computers could reliably communicate over long distances without a central control point—a fundamental principle that shaped the Internet.
The network used specialized computers called Interface Message Processors (IMPs) to route data packets between nodes. Early protocols like NCP (Network Control Protocol) evolved into TCP/IP, which became the backbone of modern networking.
Key milestones include the first message sent between UCLA and Stanford in 1969, and the adoption of TCP/IP in 1983, which marked ARPANET's transition toward what became the public Internet. The network was decommissioned in 1990, but its innovations—packet switching, distributed architecture, and open protocols—remain foundational to all modern networking.