Availability of different operating systems, hardware platforms and the geographical dispersion of the computing resources necessitated the need of networking in such a manner that computers of all sizes can communicate with each other, regardless of the vendor, the operating system, the hardware platform, or geographical proximity. Therefore, we may say that internetworking is a scheme for interconnecting multiple networks of dissimilar technologies. To interconnect multiple networks of dissimilar technologies use both additional hardware and software. This additional hardware is positioned between networks and software on each attached computer. Thus, system of interconnected networks is called an inter network or an Internet.
To find out a solution how to develop standards for internetworking, Defense Advanced Research Projects Agency (DARPA) had funded research projects in this direction. ARPA net - a project of DARPA - introduced the world of networking to protocol suite concepts such as layering, well before .ISO's initiative in this direction. DARPA continued its research for an internetworking protocol suite. This may be seen in the early NCP (Network Control Program) host-to-host protocol to the TCP/IP protocol suite, which took its current form around 1978.DARPA was well-known for its pioneering of packet switching over radio networks and satellite channels and ARPAnet was declared’ an operational network with responsibility of administering it to Defense Communications Agency (DCA) in 1975. TCP/IP had not yet been developed.
ARPAnet was basically a network based on leased lines connected by special switching nodes, known as Internet Message Processors (IMP). Many researches were involved in TCP/IP research by 1979. This motivated DARPA to form an informal committee to coordinate and guide the design of the communication protocols and architecture. The committee was called the Internet Control and Configuration Board (ICCB).
The first real implementations of the Internet may be cited from the time when DARPA started converting the machines of its research network ARPAnet to use the new TCP/IP protocols. After this transition which started in 1980 and finished in 1983, DARPA demanded that all computers willing to connect to its ARPAnet must use TCP/IP. The US military adopted TCP/IP as standard protocol in 1983 and desired that all the networks connected to the ARPAnet were required to conform to the new standards.
The success of ARPAnet was more than the expectations of its own founders and TCP/IP internetworking became widespread rapidly. As a result, new wide area networks (WAN) were created in the USA and connected to ARPAnet using TCP/IP protocol. In turn, other networks in the rest of the world, not necessarily based on the TCP/IP protocols, were added to the set of interconnected networks. Computing facilities all over worth America, Europe, Japan, and other parts of the world are currently connected to the Internet via their own sub-networks, constituting the world's largest network. In 1990, ARPAnet was eliminated, and the Internet was declared as the formal global network.
DARPA also funded project to develop an implementation of the TCP/IP protocols for Berkeley UNIX on the VAX and to distribute the necessary code developed as the outcome of this project free of charge with their UNIX operating system. The first release of the Berkeley Software Distribution (BSD) to include the TCP/IP protocol set was made available in 1983 (4.2BSD). This led to the rapid spread of TCP/IP among universities and research centers and has become the standard communications subsystem for all UNIX connectivity. Many updated versions of BSD code are available. These are: 4.3BSD (1986), 4.3BSD Tahoe (1988), 4.3BSD Reno (1990) and 4.4BSD (1993).
Some examples of the different networks that have played key roles in this development are described below:
The word Internet is a short form of a complete word inter- network or interconnected network. Therefore, this can be definitely said that the Internet is not a single network, but a collection of networks. These networks have one thing in common-to communicate with each other. That is TCP/IP. The Internet consists of the following groups of networks:
- Backbones These are large networks that exist primarily to interconnect other networks. Some examples of backbones are NSFNET in the USA, EBONE in Europe and large commercial backbones.
- Regional networks These connect, for example, universities and colleges. ERNET (Education and Research Network) is an example in the Indian context.
- Commercial networks They provide access to the backbones to subscribers, and networks owned by commercial organizations for internal use and also have connections to the Internet. Mainly, Internet Service Providers come into this category.
- Local networks These are campus-wide university networks.
The networks connect users to the Internet using special devices that are called as gateways or routers. These devices provide connection and protocol conversion of dissimilar networks to the Internet.
Gateways or routers are responsible for routing data around the global network until they reach their ultimate destination. The delivery of data to its final destination takes place based on some routing table maintained by router or gateways. These are mentioned at various places in this book as this is clear now that these are the fundamental devices to interconnect similar or dissimilar network together.
Over time, TCP/IP defined several protocol sets for the exchange of routing information. Each set pertains to a different historic phase in the evolution of the architecture of the Internet backbone.
ARPAnet was built by DARPA as described earlier. This initiated the packet switching technology in the world of networking and therefore, sometimes referred as the "grand-daddy of packet networks". The ARPA net was established in the late 1960s for the Department of Defense to accommodate research equipment on packet switching technology besides allowing resource sharing for the Department of Defense's contractors. This network includes research centers, some military bases and government locations. It soon became popular with researchers for collaboration through electronic mail and other services. ARPAnet formed the beginning of the Internet.
ARPAnet provided interconnection of various Packet-switching Nodes (PSN) located across the continental USA and Western Europe using 56 Kbps leased lines. ARPAnet provided connection to minicomputers running a protocol known as 1822 (after the number of a report describing it) and dedicated to the packet-switching task. Each PSN had at least two connections to other PSNs (to allow alternate routing in case of circuit failure) and up to 22 ports for user computer connections. Later on, DARPA replaced the 1822 packet switching technology with the CCIIT X.25 standard. Subsequent/I, with the excessive increase in the data traffic made 56 Kbps capacity of the lines insufficient. ARPAnet has now been replaced with new technologies as backbone on the research side of the connected Internet.