ARPANET

“The Advanced Research Projects Agency Network (ARPANET) was an early packet-switching network and the first network to implement the TCP/IP protocol suite. Both technologies became the technical foundation of the Internet. The ARPANET was initially founded by the Advanced Research Projects Agency (ARPA) of the United States Department of Defense. The packet-switching methodology employed in the ARPANET was based on concepts and designs by Paul Baran, Donald Davies, Leonard Kleinrock, and Lawrence Roberts. The TCP/IP communications protocols were developed for the ARPANET by Robert Kahn and Vint Cerf, and incorporated concepts from the French CYCLADES project directed by Louis Pouzin. As the network development progressed, protocols for internetworking were developed by which multiple separate networks could be joined into a network of networks. … The ARPANET project was formally decommissioned in 1990, after partnerships with the telecommunication industry paved the way of future commercialization of a new world-wide network, known as the Internet.  … Taylor had three computer terminals in his office, each connected to separate computers, which ARPA was funding: one for the System Development Corporation (SDC) Q-32 in Santa Monica, one for Project Genie at the University of California, Berkeley, and another for Multics at the Massachusetts Institute of Technology. Taylor recalls the circumstance: ‘For each of these three terminals, I had three different sets of user commands. So, if I was talking online with someone at S.D.C., and I wanted to talk to someone I knew at Berkeley, or M.I.T., about this, I had to get up from the S.D.C. terminal, go over and log into the other terminal and get in touch with them. I said, ‘Oh Man!’, it’s obvious what to do: If you have these three terminals, there ought to be one terminal that goes anywhere you want to go. That idea is the ARPANET’. … Unlike modern Internet datagrams, the ARPANET was designed to reliably transmit 1822 messages, and to inform the host computer when it loses a message; the contemporary IP is unreliable, whereas the TCP is reliable. Nonetheless, the 1822 protocol proved inadequate for handling multiple connections among different applications residing in a host computer. …”
Wikipedia
How the Internet was born: from the ARPANET to the Internet
Scientific American: Early sketch of ARPANET’s first four nodes
W – Computer Networks: The Heralds of Resource Sharing, YouTube: Arpanet, 1972
YouTube: ARPANET – The First Internet