JANET developed out of a number of local and research networks dating back to the 1970s. In the early 80s a standardization and interconnect effort started, hosted on an expansion of the pioneeringSERCnet X.25 research network. The system first went live in April 1983, hosting about 50 sites with line speeds of 9.6 kbit/s. In the mid-80s the backbone was upgraded to a 2 Mbit/s backbone with 64 kbit/s access links, and a further upgrade in the early 1990s sped the backbone to 8 Mbit/s and the access links to 2 Mbit/s, making JANET the fastest X.25 network in the world.
The JANET effort resulted in the standarization known as the Coloured Book protocols, which provided the first complete X.25 standard. There had been some talk of moving JANET to OSI protocols in the 1990s, but changes in the networking world meant this never happened.
In January 1991 the JANET IP Service (JIPS) was set up as a pilot project to host IP traffic on the existing network. Within ten months the IP traffic had exceeded the levels of X.25 traffic, and the IP support became official in November. Today JANET is primarily a high-speed IP network.
In order to address speed concerns, several hardware upgrades have been incorporated into the JANET system. In 1989 SuperJANET was proposed, to re-host JANET on a fibre optic network. Work started in late 1992, and by late 1993 the first 14 sites had migrated to the new 32 Mbit/s ATM system. SuperJANET also moved solely to IP.
In 1995 SuperJANET II started, adding 155 Mbit/s ATM backbones and a 10 Mbit/s SMDS network centered around some of the original JANET nodes. JANET's mandate now included running metropolitan area networks centered on these sites.
Today's JuperJANET III created new 155 Mbit/s ATM nodes to fully connect all of the major sites at London, Bristol, Manchester and Leeds, with 34 Mbit/s links to smaller sites around the country. JANET is connected to the equivalent academic networks in other countries and to many commercial networks in the UK and abroad forming part of the global internet.
JANET is operated by a consortia known as the United Kingdom Education and Research Networking Association (UKERNA), who are also responsible for the .ac.uk and .gov.uk domainss.
The Japanese American Network 
It can also mean "Just Another Non-Existant Terminal" when used to describe a secret military air service.