To use the services available on an Internet, application programs, running at two end computers and communicating with each other, are needed. In other words, in an Internet, the application programs are the entities that communicate with each other, not the computers or users.
The application programs using the Internet follow these client-server model strategies
1.An application program, called the client, running on the local machine, requests a service from another application program, called the server, running on the remote machine, Figure 2.12 illustrates this.

2.A server can provide a service for any client, not just a particular client. In other words, the client-server relationship is many-to-one. Many clients can use the services of one server.

3.Generally, a client program, which requests a service, should run only when it is needed. The server program, which provides a service, should run all of the time because it does not know when its service is needed.

4.Services needed frequently and by many users have specific client-server application programs. For example, we should have client-server application programs that allow users to access files, send e-mail, and so on. For services that are more customized, we should have one generic application program that allows users to access the services available on a remote computer.


A client is a program running on the local machine requesting service from a server. A client program is finite, which means it is started by the user (or another application program) and terminates when the service is complete.


A server is a program running on the remote machine providing service to the clients. When it starts, it opens the door for incoming requests from clients, but it never initiates a service until it is requested to do so.

A server program is an infinite program. When it starts, it runs infinitely unless a problem arises. It waits for incoming requests from clients. When a request arrives, it responds to the request.

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