21 November 1969: The first permanent Arpanet link

The primitive network known as Arpanet was one of the key steps in the formation of the internet. Funded by the US military, it was part of research into building a communications network that could survive a nuclear war. However, the primary goal of the project was to allow computers at research institutions to access unused capacity on each other's systems.

Arpanet's big breakthrough was the use of "packet switching" technology. Previously, networks used "circuit switching", where operators manipulated circuits to create a dedicated line for sending data. Packet switching broke down data into smaller chunks that could be sent via multiple paths and then reassembled. This was faster, more efficient and, crucially, meant if part of the network was down, data could be rerouted via a different set of connections.

Drawing partly on the the work of British computer scientist Donald Davies, who first demonstrated packet switching in 1966, Arpanet sent its first message in October 1969. The first permanent Arpanet link was set up between researchers at University California Los Angeles and the Stanford Research Institute in late November. By early December two other institutions were connected.

The development of affordable computers in the 1970s drastically reduced the need for institutions to share mainframe space. But the network continued to grow after software was developed to send electronic messages, while other networks copying the technology sprang up.

By the time Arpanet was wound up in 1990, the early form of the internet had come into existence, with 300,000 systems connected worldwide.