ARPAnet is usually coined to be the predecessor of the Internet, and indeed most of the technological heritage are from this network. ARPAnet in its first incarnation, was developed during the sixties based on ideas about packet switched networks. In packed switched networks transmission is divided into discrete packages individually sent over shared transmission lines. The ARPAnet project was founded by US Defence Advanced Research Project Agency (DARPA). During the seventies the network was expanded to include American universities, defence contractors and military institutions. In 1975 there were 57 packet switching nodes connected to this network, with peripheral nodes in Hawaii, London and Norway. In 1981 this had increased to 213 nodes. Up to four hosts could be connected to a packet switching node.
During the seventies the ARPAnet was dominated by PDP-10 computers and used a protocol stack called Network Control Program (NCP). ARPAnet was a Wide Area Network (WAN) of its own and was not an internetwork, which to days Internet is. An internetwork is a network between networks. Unix had point-to-point networking though UUCP. With UUCP, Unix users could exchange mail point-to-point which lead to the creation of servers where you could send and receive messages like on a bulletin board. This where called Usenet. UUCP formed a network separate from ARPAnet. In addition there were a chaotic number of different network technologies in the US and Europe. Some examples where the X.25 based SERCnet(1974), and CERnet(1976) with its own protocols. X.25 is an ITU-T (The standards body of International Telecommunication Union) standard protocol suite. X.25 was designed to provide a packet-switched WAN on the analog telephone system. This protocol suite is obsolete, but X.25 based networks still remain some of the only available reliable links to the Internet in many portions of the third world.
The most important reason that ARPAnet is seen as the predecessor of to days Internet is the TCP/IP protocol suite. TCP/IP was designed so that it could be used over many different kind of networks as long as the intermediate routers and end stations spoke IP. IP packets could therefore travel over may different kind of physical networks and rely on existing protocols for lower layer communication. ARPAnet switched over to TCP/IP in 1983. 4.2BSD with fully integrated TCP/IP networking where released in 1983, increasing both TCP/IP’s and Unix’s popularity. During the eighties and early nineties more and more X.25 based networks started to route IP traffic. Internet Service Providers (ISP) providing dial-up connection to IP networks replaced the networks of UUCP connected hosts. This connection of many different networks by means of IP have formed the Internet of to day.
The Open Systems Interconnection (OSI-stack) was an effort to standardise networking that was started in 1982 by the International Organization for Standardization (ISO). The OSI protocol stack was considered by many to be too complicated and was almost impossible to implement. The OSI protocol stack specified every layer in the protocol stack demanding existing protocols to be replaced. The OSI stack was eclipsed by the simpler and more pragmatic TCP/IP protocol stack. A lot of the functionality in the OSI protocol stack have later been implemented by other means using the TCP/IP stack.