In computer networking world, the computers connected to the Internet are often reffered
to as end systems. They are reffered to as end systems because they sit at the edge of the Internet.
End systems are also reffered to as host because they host (that is, run)
appllication programs such as a Web browser program, a Web server program, an e-mail reader program, or an e-mail server program.
Hosts are sometimes further divided into two categories: clients and servers.
A client program is a program running on one end system that reqquests and receives
a service from a server Program running on another end system.
Since a client program typically runs on one computer and the server program runs
on another computer, client.server Internet applications are, by definition, distributed applications. The clinet program
and the server program interact by sending each other messages over the Internet. At this level of abstraction, the routers,
links, and other nuts and bolts of the Internet serve as a blck box that transfers messagesbetween the distributed, communicating
components of an Internet application.
Not all Inter Net applications today consist of pure client programs interaction
with pure server programs.
