The Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the United States Department of Defense was the world's first operational packet switching network, and the predecessor of the global Internet.

Packet switching, now the dominant basis for both data and voice communication worldwide, was a new and important concept in data communications. Previously, data communication was based on the idea of circuit switching, as in the old typical telephone circuit, where a dedicated circuit is tied up for the duration of the call and communication is only possible with the single party on the other end of the circuit.

With packet switching, a system could use one communication link to communicate with more than one machine by assembling data into packets. 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. .

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