Ch. 3: Network Services

Network Services: Addresses and Ports

On the Internet, a client requests a network service from a server by sending a message to that server's Internet address—either an IP address or a host name will work.

Try this: in a Web browser, enter www.calvin.edu. As a result, you see the Calvin homepage, because the client software (Web browser) requested the homepage from the Web server running on the computer with the host name www.calvin.edu.
Now, in your Web browser, enter 153.105.4.23. You see the same webpage, because the Web browser (client) sent the request to the exact same computer as before; in this case, it was just unnecessary ask an intermediary DNS server to convert a host name into an IP address.

Ports
As it turns out, the addresses used by clients are even more specific than just a host name or IP address. The client not only addresses a request to a specific server; it actually addresses the request to a specific port on the server.

In the context of networking, a port can be though of in analogy to an office telephone system, which may have many extensions. You've probably heard the message "If you know your party's extension, you may dial it at any time." Similarly, a computer on a network has a variety of ports, and client requests need to be corrected to the right one. Why? Because the server is only listening on one port for a given kind of request.

For instance, a Web server usually listens for Web browser (client) requests on port 80.

A port number can be placed at the end of a URL after a colon. Try this: in your browser, enter: www-stu.calvin.edu
You will see the student organizations webpage.
Now, in your browser, enter: www-stu.calvin.edu:80
You still see that webpage, because the student Web server was indeed listening for requests on port 80.
Now, in your browser, enter: www-stu.calvin.edu:81
This generates an error, because the student Web server is not listening for requests on port 81. Rather, it is listening for requests only on port 80.

So why did it work the first time, when we didn't specify a port number at all? This is because Web browsers are programmed to send all requests to port 80 automatically unless you specify otherwise.

Ports are obviously very important inroads into your computer system. For this reason, one of the techniques crackers use is called "port scans"—i.e., they scan all of the ports to see if there's an insecure one through which the cracker can break in to the computer.

This is one of the roles of a firewall, which is hardware or software that protects individual computers or LANs from improper access via the Internet.

Your home computer has ports too: be careful, especially if you leave your computer connected to the Internet for long periods of time. (An excellent and free firewall software program called ZoneAlarm is produced by Zone Labs.)

If you encounter technical errors, contact computing@calvin.edu.