Before the Internet

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In the 1950s and early 1960s, prior to the widespread inter-networking that led to the Internet, most communication networks were limited in that they only allowed communications between the stations on the network. Some networks had gateways or bridges between them, but these bridges were often limited or built specifically for a single use. One prevalent computer networking method was based on the central mainframe method, simply allowing its terminals to be connected via long leased lines. This method was used in the 1950s by Project RAND to support researchers such as Herbert Simon, at Carnegie Mellon University in Pittsburgh, Pennsylvania, when collaborating across the continent with researchers in Sullivan, Illinois, on automated theorem proving and artificial intelligence.
Three terminals and an ARPfb
Main articles: RAND and ARPANET
A fundamental pioneer in the call for a global network, J.C.R. Licklider, articulated the ideas in his January 1960 paper, Man-Computer Symbiosis.
"A network of such [computers], connected to one another by wide-band communication lines [which provided] the functions of present-day libraries together with anticipated advances in information storage and retrieval and [other] symbiotic functions."
—J.C.R. Licklider, [2]
In October 1962, Licklider was appointed head of the United States Department of Defense's Advanced Research Projects Agency, now known as DARPA, within the information processing office. There he formed an informal group within DARPA to further computer research. As part of the information processing office's role, three network terminals had been installed: one for System Development Corporation in Santa Monica, one for Project Genie at the University of California, Berkeley and one for the Compatible Time-Sharing System project at the Massachusetts Institute of Technology (MIT). Licklider's identified need for inter-networking would be made obvious by the apparent waste of resources this caused.
"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, it's obvious what to do (But I don't want to do it): If you have these three terminals, there ought to be one terminal that goes anywhere you want to go where you have interactive computing. That idea is the ARPAnet."
—Robert W. Taylor, co-writer with Licklider of "The Computer as a Communications Device", in an interview with the New York Times, [3]
Packet switching
Main article: Packet switching
At the tip of the internetworking problem lay the issue of connecting separate physical networks to form one logical network. During the 1960s, Paul Baran (RAND Corporation), produced a study of surviveable networks for the US military. This was based on small 'message-blocks' Donald Davies (National Physical Laboratory, UK), proposed and developed a network based on packet-switching, a technology which had also been studied and analysed by Leonard Kleinrock (MIT). Packet-switching provided better bandwidth utilization and response times than the traditional cicuit-switching technology used for telephony, particularly on resource-limited interconnection links.
Packet switching is a rapid store-and-forward networking design that divides messages up into arbitrary packets, with routing decisions made per-packet. Early networks used message switched systems that required rigid routing structures prone to single point of failure. This led Paul Baran's US Military funded research to focus on using message-blocks to include network redundancy,[4] which in turn led to the widespread urban legend that the Internet was designed to resist nuclear attack