ARPANET or Arpanet Definition/Meaning:
Acronym for Advanced Research Projects Agency Network. The
collection of host computers plus backbone network making up the first major packet-switching network. Begun through research grants provided by ARPA (now
DARPA) to universities and private organizations for computer science, the
ARPANET started as a four-node network in December 1969. It now connects over
100 host computers throughout half the world.
The ARPANET utilizes minicomputers called interface message processors (IMPs) as a backbone network of
message-switching nodes. The IMPs are primarily connected by 56K bps leased
lines, although other links are also used (such as the satellite link between
the US and Europe). Each IMP takes blocks of data from its hosts, subdivides
them into 128 byte packets, and adds a header specifying destination and source
addresses. Then, based on a dynamically updated routing table, the packet is
routed over whichever line is currently the fastest route to the destination.
Upon receiving a packet, the next IMP acknowledges it and repeats the routing
The ARPANET pioneered many of the network concepts that are in use today. An
important contribution is the ARPANET routing algorithm, which is completely
distributed, is performed on a packet-by-packet basis, and is based on a
continuous evaluation of the network's topology, capacity, and utilization. The
practical demonstration of distributed routing was viewed as an important step
in demonstrating that packet switching networks would be cost-effective and
ARPANET technology has been successfully replicated for several other
military and intelligence-gathering networks used by the US. The ARPANET itself
was transferred to the Defence Communication Agency in 1975, and is no longer
the direct responsibility of DARPA.