ARPANET (Advanced Research Projects Agency Network) was the pioneering wide-area packet-switched network commissioned by the Advanced Research Projects Agency (ARPA, later DARPA) of the United States Department of Defense. Its intellectual foundations lay in the packet-switching theory developed independently by Paul Baran at the RAND Corporation (1964), Donald Davies at the U.K. National Physical Laboratory, and Leonard Kleinrock at MIT. J.C.R. Licklider's 1962 memoranda on an "Intergalactic Computer Network" and the program leadership of Lawrence Roberts shaped its design. The network became operational on 29 October 1969, when the first host-to-host message was sent from Leonard Kleinrock's laboratory at UCLA to the Stanford Research Institute (SRI); the system crashed after transmitting "LO" of the intended word "LOGIN." The Interface Message Processors (IMPs), built by Bolt, Beranek and Newman (BBN), served as the early packet routers.
The defining feature of ARPANET was packet switching, which broke data into discrete packets routed independently across a distributed network—a design valued for resilience and efficient line-sharing, contrasting with the circuit-switching of contemporary telephone systems. The early network used the Network Control Protocol (NCP). The decisive evolution came with the development of the Transmission Control Protocol/Internet Protocol (TCP/IP) suite by Vinton Cerf and Robert Kahn, first described in their 1974 paper. On 1 January 1983—"flag day"—ARPANET formally switched from NCP to TCP/IP, an event widely regarded as the technical birth of the Internet, since TCP/IP enabled internetworking among heterogeneous networks. Ray Tomlinson introduced networked email in 1971 and popularized the "@" symbol for addressing.
By the late 1970s ARPANET connected dozens of research and military sites. In 1983 the military traffic was split off into MILNET, leaving ARPANET as a research network. The National Science Foundation's NSFNET (1985) progressively superseded it as the backbone, and ARPANET was formally decommissioned in 1990. Its protocols, culture of open standards (the Request for Comments or RFC series began in 1969), and architectural principles survive directly in today's Internet. ARPANET demonstrated the viability of distributed digital communication and seeded the institutions—and many of the people—who built the global network.
For the FSOT and allied civil-service examinations, ARPANET appears under job knowledge and general/world history sections testing the evolution of communications technology and U.S. government-funded innovation. Typical question angles ask for the year of first operation (1969), the sponsoring agency (ARPA/DARPA, Department of Defense), the originating institution (UCLA) and first node-pair (UCLA–SRI), the inventors of TCP/IP (Cerf and Kahn), and the 1983 transition that marks the Internet's technical origin. Candidates should distinguish ARPANET (the network) from the Internet (the global system of interconnected networks) and from the World Wide Web (Tim Berners-Lee's 1989–1991 hypertext application running atop the Internet). Awareness of the Cold War defense rationale—and the common myth that it was designed solely to survive nuclear attack—adds analytical depth.
Example
On 29 October 1969, Leonard Kleinrock's team at UCLA transmitted the first ARPANET message to the Stanford Research Institute, sending "LO" before the system crashed mid-word.
Frequently asked questions
ARPANET was a single packet-switched research network sponsored by the U.S. Defense Department from 1969. The Internet is the global system of interconnected networks that emerged after ARPANET adopted TCP/IP in 1983, enabling internetworking among many heterogeneous networks.