Clients and Servers

A network application protocol typically has two parts or "sides", a client side and a server side. The client side in one end system communicates with the server side in another end system. For example, a Web browser implements the client side of HTTP and a Web server implements the server side of HTTP. In another example, e-mail, the sending mail server implements the client side of SMTP and the receiving mail server implements the server side of SMTP.
For many applications, a host will implement both the client and server sides of an application. For example, consider a Telnet session between Hosts A and B. (Recall that Telnet is a popular remote login application.) If Host A initiates the Telnet session (so that a user at Host A is logging onto Host B), then Host A runs the client side of the application and Host B runs the server side. On the other hand, if Host B initiates the Telnet session, then Host B runs the client side of the application. FTP, used for transferring files between two hosts, provides another example. When an FTP session exists between two hosts, then either host can transfer a file to the other host during the session. However, as is the case for almost all network applications, the host that initiates the session is labeled the client. Furthermore, a host can actually act as both a client and a server at the same time for a given application. For example, a mail server host runs the client side of SMTP (for sending mail) as well as the server side of SMTP (for receiving mail).