ARPANET (Advanced Research Projects Agency Network), created by a small research team at the head of the Massachusetts Institute of Technology and the Defense Advanced Research Projects Agency (DARPA) of the United States Department of Defense, was the world's first operational packet switching network, and one of the networks that came to compose the global Internet. The packet switching of the ARPANET was based on designs by Lawrence Roberts, of the Lincoln Laboratory.

Packet switching, now the dominant basis for data communications worldwide, then was a new and important concept. Data communications had been based on the idea of circuit switching, as in the old, typical telephone circuit, wherein a dedicated circuit is occupied for the duration of the telephone call, and communication is possible only with the single party at the far end of the circuit.

With packet switching, a data system could use one communications link to communicate with more than one machine by disassembling data into datagrams, then gather these as packets. Thus, not only could the link be shared (much as a single post box can be used to post letters to different destinations), but each packet could be routed independently of other packets.