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The Advanced Research Projects Agency Network (ARPANET) was one of the main precursors to the Internet. At the time it was conceived, most communications were thought of in terms of circuit switching, but ARPANET implemented the type of packet switching network that would come to dominate communications around the world. The network was based on the principle of creating small data packets that could be sent across the network link at any time it was available, rather than securing a circuit, like would be required in a circuit switching network.
ARPANET began as a project of the US Department of Defense (DoD). It was initially developed by teams at the Defense Advanced Research Projects Agency (DARPA) and the Massachusetts Institute of Technology (MIT). The ideas behind a packet switched network dated to the early 1960s, though the Defense Advanced Research Projects Agency's (DARPA's) network itself wasn't designed and implemented until the latter part of that decade.
The first computers linked through ARPANET were located at Stanford University, the University of Utah and two University of California campuses. Each campus had a computer known as an interface message processor (IMP) that functioned similar to a modern day router. They each then connected both a Scientific Data Systems (SDS) and International Business Machines (IBM) or Digital Equipment Corporation (DEC) computer to the IMP, and through it to ARPANET. These four nodes comprised the entire network in late 1969.
In order to reliably communicate data in a bidirectional manner, the network control program (NCP) was designed. This effectively allowed various applications to communicate through the network. Various applications protocols were created throughout the 1970s. Email was introduced on the network in 1971, and file transfer protocol (FTP) was implemented in 1973. A voice transfer protocol, known as network voice protocol (NVP), was also created, though network bandwidth hindered its use.
In 1983, NCP was replaced by the modern transmission control protocol/internet protocol (TCP/IP) when ARPANET integrated with other networks to form the early Internet. Many of the existing application protocols, such as FTP, were carried over to the suite of protocols used by the Internet. Also in 1983, US military portions of the network were split off to create the discrete military network (MILNET).
ARPANET operated continuously from 1969 until it was decommissioned in 1990. The last IMPs went offline in the late 1980s, and ARPANET was merged together with a number of other networks to form the backbone of what would come to be known as the Internet. These networks utilized the common TCP/IP protocol in addition to a number of application protocols first introduced on ARPANET.