This configuration is called the client/server model. In the client/server model, one program requests a service from another program.
The client makes a request; the server answers the request with a service.
The client and server communicate with each other by both physical (hardware) and logical (software) links.
A client consists of:
A desktop computer (hardware)
A program (software) that requests information from the server
The program usually resides on a personal computer.
An example of such a program is a Web browser. When you want to see a particular Web site, the browser handles your request.
Typically a server is a remote device shared by many users. A server can be:
A machine that stores a particular resource you want to use, such as a database or Web pages
A machine that performs specific functions such as sending and receiving e-mail
Servers are available commercially from companies including Sun Microsystems, Sequent, IBM, Unisys, Dell, Compaq, and NCR.
The figure below illustrates the relation between a client and server connected through local and regional Internet Service Providers (ISPs).
The Internet is a connection of individual networks, where the individual networks are owned by a huge number of independent operators. Some of the individual networks are major corporations with large, high-capacity networks, while others are private individuals operating small networks of consisting of several LANs in a small building. Between them these networks employ just about every networking technology yet invented. The great strength of the Internet is that it allows these diverse networks to act together to provide a single global network service.
The interactions between a network and neighboring networks are both simple and robust. This makes for easy extendibility and fuelled the early growth of the Internet. New participants needed only to come to an agreement with an existing operator and set up some fairly simple equipment to become full players. This was in great
contrast to the situation within the world of telephone networks, where operators were mostly large and bureaucratic and where adding new interconnections required complex negotiation and configuration and, possibly, international treaties.