Interface Message Processor (IMP)

Interface Message Processor (IMP)

The Interface Message Processor (IMP) is an early packet-switching device which played a crucial role in developing the ARPANET, the precursor to the modern internet. IMPs were the first-generation nodes of the ARPANET, acting as the backbone for data transmission and facilitating communication between connected computers.

The Interface Message Processor was the packet-switching node used to interconnect participant networks to the ARPANET from the late 1960s to 1989. It was the first generation of gateways, which are known today as routers.[1]

Purpose and Role

The primary purpose of the IMP was to route and manage data packets between connected computers on the ARPANET. It provided a reliable and efficient communication by breaking down messages into smaller packets, transmitting them over the network, and reassembling them at the destination.


The IMP was a specialized computer, typically a Honeywell DDP-516 or 316 minicomputers, with custom hardware and software for packet-switching. Key components included:

  1. Processor: The central processing unit (CPU) that executed the IMP software, handling the routing and management of data packets.
  2. Memory: Storage used to buffer incoming and outgoing packets, as well as to store routing tables and other network information.
  3. Interfaces: Hardware interfaces that connected the IMP to the ARPANET and to the host computers at each site.
  4. Software: The IMP's operating system and packet-switching software, which implemented routing algorithms, error detection and correction, and other networking functions.


The IMP's importance lies in its pioneering role in the development of the internet:

  1. Packet-switching: The IMP was among the first devices to implement packet-switching technology, a fundamental concept that underpins today's internet.
  2. ARPANET: As the backbone of the ARPANET, IMPs were crucial in enabling the first wide-area computer network, paving the way for future network technologies.
  3. Reliability: The IMP's robust design and error-handling capabilities contributed to the stability and resilience of the ARPANET, setting high standards for future networking equipment.


The development of the IMP began in 1968, when the U.S. Department of Defense's Advanced Research Projects Agency (ARPA) awarded a contract to Bolt, Beranek, and Newman (BBN) to build the first four IMPs. The first IMP was installed at the University of California, Los Angeles (UCLA) in 1969, marking the birth of the ARPANET. Over time, more IMPs were installed at other institutions, and the ARPANET grew, eventually evolving into the internet we know today.


The Interface Message Processor's legacy is its foundational role in developing the internet. The concepts and technologies pioneered by the IMP, such as packet-switching and network reliability, continue to influence modern networking devices and protocols, shaping how we communicate and share information online.

See Also


  1. Defining Interface Message Processor (IMP) Wikipedia