The Interface Message Processor (IMP) was a hardware device - built by Bolt, Beranek and Newman (BBN) technologies - that was an integral component of the architecture of ARPANET. Plans to build ARPANET were finalised in 1968 by Larry Roberts - they had begun in 1967 at the "ARPANET Design Session" in Ann Arbor, Michigan - and procurement to build the (wide area) network was initiated in 1968. Wesley A. Clark - who created the first minicomputer - suggested to Larry Roberts that he should place a minicomputer in-between the host computers of each network connected to ARPANET. Wesley A. Clark believed these minicomputers, later to be called IMPs, would reduce the work load placed on the host computers, and would provide an interface to ARPANET. The host computers - which used different operating systems - would only have to design an interface to connect to the IMP; simplifying the process of connecting host computers to ARPANET.
Leonard Kleinrock standing next to ARPANET's Interface Message Processor (IMP)
BBN technologies won the contract to build the hardware (IMPs) necessary to implement ARPANET. ARPANET was to be the first computer network in the US to use packet switching, and the protocols and design architecture of ARPANET would evolve and be used on the Internet. A team of computer scientists and engineers from BBN met the team from ARPA who would design the protocols for ARPANET in 1969. The BBN team would include Robert Kahn, who would coinvent TCP/IP in the early 1970's. TCP/IP is the core protocol suite of the Internet. The result of the procurement procedure and meetings with ARPA was the development of Interface Message Processor's; referred to as IMPs. IMPs would 'serve' as a node on ARPANET, and would transmit and receive data packets from other nodes connected to ARPANET.
The BBN team which designed and built the IMP was led by Frank Heart. IMPs included a 16-bit minicomputer that was built by Honeywell. IMPs were used on ARPANET until 1989, and IMPs evolved to use a range of Honeywell minicomputer models, such as: 316, 416, 516 and 716. The first 316 Honeywell minicomputers were programmed by W.Crowther, D.Walden and B.Cosell, and a summary of the IMP software can be found in the first Request for Comments document:
The first IMP was delivered to UCLA on the 30th of August, 1969. There would initially be four IMPs built for ARPANET: the remaining IMPs were delivered to the Stanford Research Institute, the University of Utah and the University of California. Therefore, to being with, ARPANET would be a wide area network comprising four nodes. The host computer at each university - that the IMP would be connected to - was different and would be connected using a serial interface.