An agency of the United States Department of Defense, ARPA (Advanced Research Projects Agency) underwrote development for the precursor of the Internet, known as ARPANET. Initially a modest network of four interconnected university computers, ARPANET's initial purpose was to enable mainly scientific users at the connected institutions to communicate and share resources. 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.
Because ARPA's name was changed to Defense Advanced Research Projects Agency (DARPA) in 1971, some people refer to ARPANET as DARPANET. (DARPA was changed back to ARPA in 1993 and back to DARPA again in 1996.)