What Is ARPANET?

ARPANET (Advanced Research Projects Agency Network) was the first wide-area packet-switching network and the direct precursor of the modern internet. Commissioned by the United States Defense Advanced Research Projects Agency (DARPA) and made operational on October 29, 1969, ARPANET initially connected four nodes: the University of California Los Angeles, the Stanford Research Institute, the University of California Santa Barbara, and the University of Utah. The network was designed to allow geographically dispersed research computers to share resources and exchange data, replacing the point-to-point circuit-switched telephone model with a fundamentally different approach in which messages travel as independently routed data packets. ARPANET was formally decommissioned in 1990 after its core technologies had migrated into the civilian internet.

The network grew from a recognition, formalized by researchers including J.C.R. Licklider and later Robert Taylor, that the computational resources of major universities and defense laboratories could be made far more productive if researchers could access remote machines and datasets without physically traveling to each site. The technical challenge was to build a communications infrastructure that would remain functional even if individual nodes failed, a requirement that circuit-switched telephone networks did not satisfy.

Packet Switching Architecture

The central technical innovation of ARPANET was packet switching, a concept developed independently by Paul Baran at RAND Corporation and Donald Davies at the UK National Physical Laboratory. Rather than establishing a dedicated end-to-end circuit for each communication session, packet switching breaks a message into fixed-length blocks called packets, each carrying a header with source and destination addresses. Packets from different conversations share the same physical links, and each intermediate node (called an Interface Message Processor, or IMP) forwards packets toward their destination based on routing tables that can adapt to network conditions. The DARPA historical account of ARPANET describes how this design allowed the network to route around damaged or congested nodes automatically, fulfilling the resilience requirement that motivated the project. BBN Technologies, contracted in 1968, built the original IMPs using Honeywell DDP-516 minicomputers.

Network Protocols and Growth

The first operational protocol on ARPANET was the Network Control Protocol (NCP), which governed host-to-host communication and assumed that the underlying network was reliable, a limitation that became apparent as ARPANET grew. Vint Cerf and Robert Kahn addressed this limitation by developing the Transmission Control Protocol and Internet Protocol (TCP/IP), first described in a 1974 paper and implemented operationally at Stanford by 1975. TCP divided messages into segments, provided end-to-end reliability and flow control, and allowed heterogeneous networks to interconnect, while IP handled addressing and packet routing between networks. By the early 1980s, ARPANET had expanded from its original four nodes to several hundred host computers, connecting universities, defense contractors, and government laboratories. The Internet Society's history of the internet documents this period in detail, noting that ARPANET served as the backbone across which the new TCP/IP protocol suite was tested and refined at scale.

Legacy and Transition to the Internet

In January 1983, ARPANET completed its transition to TCP/IP, an event sometimes called "flag day." At the same time, the network was split into ARPANET (serving the research community) and MILNET (serving military sites), with gateways connecting the two. The coexistence and eventual merger of ARPANET with other TCP/IP networks gave rise to what is now called the internet. ARPANET was officially shut down on February 28, 1990, by which time the commercial internet had absorbed its function entirely. The technical vocabulary it introduced, including packet, routing table, host, and gateway, remains standard in networking.

Applications

ARPANET established the technical foundations for several categories of networked services, including:

• Electronic mail, first transmitted over ARPANET in 1971
• Remote login and file transfer (Telnet and FTP protocols)
• Distributed resource sharing across research institutions
• Development and testing of TCP/IP, the protocol suite underlying the modern internet infrastructure