Clients and Servers

Client and server describes the relationship between two computer programs in which one program, the client, makes a service request from another program, the server, which fulfils the request. Although the client/server idea can be used by programs within a single computer, it is a more important idea in a network. In a network, the client/server model provides a convenient way to interconnect programs that are distributed efficiently across different locations. Computer transactions using the client/server model are very common. 

Internet servers make the Internet possible. All of the machines on the Internet are either servers or clients. The machines that provide services to other machines are servers. And the machines that are used to connect to those services are clients. There are Web servers, e-mail servers, FTP servers and so on serving the needs of Internet users all over the world. 

When you connect to www.yahoo.com to read a page, you are a user sitting at a client's machine. You are accessing the Yahoo Web server. The server machine finds the page you requested and sends it to you. Clients that come to a server machine do so with a specific intent, so clients direct their request to a specific software server running on the server machine. For example, if you are running a Web browser on your machine, it will want to take to the Web server on the server machine, not the e-mail server.

A server has a static IP address that does not change very often. A home machine that is dialling up through a modem, on the other hand, typically has an IP address assigned by the ISP every time you dial in. That IP address is unique for your session, it may be different the next time you dial in. This way, an ISP only needs one IP address for each modem it supports, rather than one for each customer.