Taken from the original at http://www.ecst.csuchico.edu/~beej/guide/net/ ... see end of writeup for Copyright statement.
It's a client-server world, baby. Just about everything on the network deals with client processes talking to server processes and vice-versa. Take telnet, for instance. When you connect to a remote host on port 23 with telnet (the client), a program on that host (called telnetd, the server) springs to life. It handles the incoming telnet connection, sets you up with a login prompt, etc.
The exchange of information between client and server is summarized in this figure:
+------> information -->--+
| returned |
+----<-- information <----+
Note that the client-server pair can speak SOCK_STREAM, SOCK_DGRAM, or anything else (as long as they're speaking the same thing.) Some good examples of client-server pairs are telnet/telnetd, ftp/ftpd, or bootp/bootpd. Every time you use ftp, there's a remote program, ftpd, that serves you.
Often, there will only be one server on a machine, and that server will handle multiple clients using fork(). The basic routine is: server will wait for a connection, accept() it, and fork() a child process to handle it. This is what our sample server does in the next section.
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Copyright © 1995, 1996 by Brian "Beej" Hall. This guide may be reprinted in any medium provided that its content is not altered, it is presented in its entirety, and this copyright notice remains intact. Contact email@example.com for more information.