ARPANET was the network that became the basis for the Internet. Based on a concept first published in 1967, ARPANET was developed under the direction of the U.S. Advanced Research Projects Agency (ARPA). In 1969, the idea became a modest reality with the interconnection of four university computers. The initial purpose was to communicate with and share computer resources among mainly scientific users at the connected institutions. ARPANET took advantage of the new idea of sending information in small units called packets that could be routed on different paths and reconstructed at their destination. The development of the TCP/IP protocols in the 1970s made it possible to expand the size of the network, which now had become a network of networks, in an orderly way.
In the 1980s, ARPANET was handed over to a separate new military network, the Defense Data Network, and NSFNet, a network of scientific and academic computers funded by the National Science Foundation. In 1995, NSFNet in turn began a phased withdrawal to turn the backbone of the Internet (called vBNS) over to a consortium of commercial backbone providers (PSINet, UUNET,ANS/AOL, Sprint, MCI, and AGIS-Net99).
Because ARPA's name was changed to Defense Advanced Research Projects Agency (DARPA) in 1971, ARPANET is sometimes referred to as DARPANET. (DARPA was changed back to ARPA in 1993 and back to DARPA again in 1996.) The history of ARPANET and developments leading up to today's Internet can be found in Where Wizards Stay Up Late, by Katie Hafner and Matthew Lyon.