The internet grew out of ARPANET, a research network funded by the United States Department of Defense that connected its first four university computers in 1969. Its key idea was packet switching, in which messages are broken into small pieces that can travel by different routes and be reassembled at their destination, making the network robust even if parts of it failed. In the 1970s, Vint Cerf and Robert Kahn designed the TCP/IP protocols, which allowed separate networks to talk to one another, and this common language was adopted across ARPANET on the first day of 1983. Email quickly became the most popular application, and universities and research labs around the world joined.
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