Introductions:- Although the computer industry is young compared to other industries, computers have made spectacular progress in a short time. During the first two decades of  their existence, computer systems were highly localised, usually within a single large room. Not infrequently this room had glass walls, through which visitors could gaze at the great electronic wonder inside. The idea that within 40 years equally powerful computers smaller than postage stamps would be produced by the millions was a pure science fiction.
                 As computers have become smaller cheaper and more numerous, people have become more and more interested in connecting them together to from networks. At first these connections were made in an adhoc way,  typically with each computer regarding the other ones as terminals. Howevet in the past decade a substantial body of knowledge has develiped on an subject of computer network, so that future networks and distributed system can be designed systematically.
                      The merging of computers and communications is total and complete. The two technologies are complementary to each other. They use the same microelectronics as their building blocks. Though the communication and computer industries evolved independently yet they have blended in exorably both technically and more recently organisationally. Blending of computers and telecommunication technologies will allow more information to be transmitted in a lesser time and at a lower cost. This has profound influence on the way computer systems are organised.
Benefits of computer Networks:- Now we will see why people are intersted in computer network and what they can be used for. This can be discussed under the headings, network goals and applications of networks.
NETWORK GOALS:-
Desire to end the tyranny of Geography:- This goal is to make all programs, data and other resources available to any one on the network without regard to the physical locations of the resource and user. In other words, the mere fact that a user happens to be many thousands km. Away from his data should not prevent him from using the data as though it were local. Load sharing is another aspect of resource sharing. Geographically the networks can be classified into following categories:-
1. LAN
2. WAN
C. INTER WORKING
Graceful Degradation upon Failure:- By having alternative source of simply, the computer networks provide high reliability with unconnected computers. If a machine goes down due to hardware failure, for example, the local users are out of circle even though there may be substantial computing capacity available elsewhere. With a network the temporary loss of a single computer is much less serious because its users can often be accomodated elsewhere until the service is restored. A complete loss of computing power for even a few hours due to some reason natural or otherwise is completely intolerable. In other words loss of 1% of the processors should result in a slightly diminished performance for everyone; rather than leaving 1% of the users in the cold.
1. Cost of computing verses communications:- Until 1970, computers were relatively expensive compared to communication facilities. The reverse is now true. Now the cost of small computer is negligible. So it becomes attractive to analyze the data at the place where it is captured and only send occasional summaries back to the computer centre to reduce the communication cost, which now represents a large percentage of the total cost than it used to.
2. Communication Media:- Yet another goal of setting up a computer network has little to do with at all. As a side effect of its others goals a computer network can provide a powerful communication medium among widely separated people. In the long run the use of networks to enhance human-to- human communication may prove more important than technical goals such as unproved reliability.
3. Superior Price/Performance Ratio:- Main frames are roughly a factor of I/O faster than the largest single chip microprocessors but they cost a thousand times more. This imbalance has caused many system designers to try to harness a collection of microcomputers to outperform the large mainframes at lower costes.
4. Incremental Growth:- Ability to increase system performance gradually at the workload grows by just adding more processors.
5. Simpler Software Design:- Another major attraction of building large systems by coupling large numbers of smaller processors is the expectation of a simpler software design. In a network, it is possible to dedicate some or all of the processors to specialised functions for example database management. Instead of having the machines by multiprogrammed (time shared) each machine does only one thing at a time. By eliminating the multiprogramming, we can also eliminate much of the software complexity associated with the large mainframes.
Applications of computer Network:- Computer networks makes many new applications feasible. Some of these new applications will use networking for economic reasons. Calling up a distant computer in a network is cheaper than calling it directly. Therefore these may have important effect on society as a whole. To give an idea about some possible future uses of computer networks, we will now briefly look at some of the examples:-
A. [ACCESS TO REMOTE PROGRAMS]
Some of the applications envisaged are:-
1. Run large programs:- Log-in over the network to run a large programs written in an exotic language, machine dependent or embedded in a web of libraries, special system calls and other non-portable environmental features. Simulation programs often fall into this category.
2. Computer Aided Education
3. Medical diagnosis
4. Tax saving
5.  Sophisticated Games.
6. Recreational Activities and so on.
            In short almost any program offering a useful service to any organisation big or small or even to an individual which is too complicated is a potential candidate for access in a network.
B. [ACCESS TO REMOTE DAYABASE]
1. Reservations:- For aeroplanes, train, buses, hotels, restaurants, theateres, while sitting at home with instant confirmation.
2. Home banking
3. Automated newspaper.
4. Full automated library.
C. [VALUE AIDED COMMUNICATION FACILITIES]
This feature includes communication media which contains:-
1. Postal services
2. Tele conferencing
3. International contacts by human beings.
4. Playing bridge with a friend in England.
5. Learning languages:- Children may' learn other languages by communication, possibly with the help of closed-circuit television which in principle can also be sent over a computer network just as a digitised voice can.
        It is sometimes said that there is a race going on between transported and communication and whichever one will win make the other unneccessary. Using a computer network as a sophisticated communications system may reduce the amount of travelling done, thus saving energy. Home work may become popular especially for part time workers with young children. The office and school as we know them now may disappear, stores may be replaced by electronic mail order catalog. Cities may disperse, since high quality communication facilities tend to reduce the need for physical proximity. The information revolution may change society as much as the industrial revolution did.