A two-network TCP/IP communications test was performed between Stanford and University College London (UCL), In 1975. In November, 1977, a three-network TCP/IP test was conducted between the U.S., UK, and Norway. Between 1978 and 1983, several other TCP/IP prototypes were developed at multiple computers research centres.

The full switchover to TCP/IP was performed on January 1, 1983, without too many problems, although a few recalcitrant sites were down as long as three months while they retrofitted their systems.

The TCP/IP protocol was deployed to the ARPANET network with some persuasion, as noted in the emails at the top of the page. Not all sites were preparing to convert over their protocols, so Cerf, Jon Postel, and the TCP/IP team turned off the NCP network channel numbers on the ARPANET IMP’s for a full day in mid 1982, so that only sites using TCP/IP could still operate. To emphasize the point, they disabled NCP again for two days later that fall.

In 1984, the US Department of Defense made TCP/IP the standard for all military computer networking, which gave it a high profile and stable funding.

[livinginternet.com, wikipedia.org]