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Computer Networks/Networks

Notes 9.4 Internetworking

The availability of different operating systems, hardware platforms and the geographical
dispersion of computing resources necessitated the need of networking in such a manner that
computers of all sizes could 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.
Some of the factors that make networks differ are frame, packet, and message size, checksum
algorithms, maximum packet lifetimes, connection-oriented vs. connectionless protocols, timer
values, etc. When all routers in a network have same protocols then the network is called
homogeneous. When these homogeneous networks are interconnected, it results in an
internetwork. There may be instances, when the routers of different networks use different
protocols such as Internet Protocol (IP), Systems Network Architecture (SNA), Asynchronous
Transfer Mode (ATM), Novel NCP/IPX and AppleTalk are some. Wireless ad-hoc and mobile
networks using e.g. Bluetooth are others. Hence, to interconnect multiple networks of dissimilar
technologies use of both additional hardware and software is needed. This additional hardware
is positioned between networks and software on each attached computer. This system of
interconnected networks is called an internetwork or an Internet.
An Example Internetwork

Figure 9.5: Internetworking of Different Homogeneous Networks

* Shades represents homogeneous networks of different types
To develop standards for internetworking, Defense Advanced Research Projects Agency (DARPA)
funded research projects. ARPAnet, a project of DARPA, introduced the world of networking
with 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 researchers 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 implementation of the Internet was when DARPA converted 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

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