ARPANET

ARPANET was an early packet-switching network funded by the U.S. Department of Defense's Advanced Research Projects Agency (ARPA) that became the technical core of the Internet. It was designed to enable resource sharing among researchers and provide resilience against outages by distributing communication control among multiple sites.

Launched in 1969, ARPANET linked four initial hosts—the University of California, Los Angeles; Stanford Research Institute; University of California, Santa Barbara; and the University of Utah. Each site used Interface Message Processors built by BBN to route packets across long distances.

In the 1970s the network expanded and introduced email in 1971; by 1973 Vinton Cerf and Robert Kahn led the development of Transmission Control Protocol/Internet Protocol (TCP/IP). In 1983 ARPANET formally adopted TCP/IP, and the network began using TCP/IP as its standard communications protocol. The same era saw the creation of the Domain Name System.

During the 1980s ARPANET grew to connect many universities and research centers; MILNET separated for military use, with ARPANET continuing to carry civilian and research traffic until its decommissioning in 1990.

ARPANET's legacy includes the foundational technologies and networking practices that became the Internet: packet switching, host-to-host communications, TCP/IP, and network protocols; many early researchers and institutions contributed to its development.