Ports

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Servers that communicate over IP listen to various IP ports to find out what to do. An FTP server constantly listens to port 21 (by default) for requests. The default port for HTTP is 80, meaning that a Web server listens to that port, waiting for requests to come along. You can change this port, but that's usually not a good idea, since applications assume they can send you an HTTP GET or POST request to port 80 and have your server process it in a known way.

Another form of HTTP, called HTTPS, provides secure encryption between a Web browser client and your Web server. A Web server that is HTTPS-compliant listens at port 443 by default. Web servers that have secure encryption capability will communicate with the Web browser by sending encryption codes when a user requests a secure connection. Every time the client and server exchange data, the sender encrypts the data and the receiver decrypts the data, all according to the encryption keys that they exchange.

Since HTTPS is more complex than HTTP, I don't want to spend more time on it. Just know that the client and server can negotiate encrypted packets in a manner that is transparent to the user and downstream processes.

Other services use other default ports. For example, the RealServer from RealNetworks uses ports 554 and 7070 (and port 80 if either of these is not available), and Telnet uses port 23. Network administrators can use firewalls and proxy servers to restrict access to certain information types by simply blocking packets targeted at particular ports.