TCP/IP networking is the most prevalent protocol in use today, and dealing with ports is a common task. So, what is a port? Here's a basic explanation. Simply put, a port is a line of communication between two computers. Every computer on a network or on the Internet has an IP address. For internal networks, typically each computer is assigned an IP address like 192.168.0.1, 192.168.0.2, or 192.168.0.3, etc... So data travels between computers and the packets of data have an IP address of the destination computer that should receive the data. It's almost like writing an address on an envelope. Once the computer receives the data, it figures out which port to send the data to. Ports are like channels on a TV or radio stations. Someone has to be listening on that specific channel or port, otherwise the data is discarded. Ports are numbered 1 to 65535 (2^16).
The listener is usually a software application that needs to receive data over network or the Internet. So for example, a web server would be listening on port 80 for people who want to surf the web site it's hosting. The browser on the user side would specify a web address like http://www.yahoo.com and it would get translated into an IP address like 215.135.85.2 (fictitious), and data would be sent to that computer which has a web server running and listening on port 80. So a line of communication will be established between the client computer and the server over this port 80, and the client would initiate the communication and request that it would like to see the yahoo home page.
Browsers assume that web servers are listening on port 80 because this is a standard. In fact, the first 1000 ports are reserved for specific purposes. You could write software that communicates on these ports, but you're really not supposed to, and it might interfere with other software running on the computer. For example port 110 is used for POP3 email, and 25 is used for SMTP.
You could run a web server on a different port if you want. However the browser would have to specify explicitly to send the data on a different port because browsers are programmed to work on port 80 by default. The most common example of a web server listening on a different port is your home wireless router which listens on port 8080 by default for most brands. Typically these routers have web based configuration screens, meaning there is a simple web server built into the router for the sole purpose of letting users configure the settings using forms viewed through your browser. To access these configuration pages you'd need to specify the port explicitly like this: http://192.168.0.1:8080. Notice the :8080 on the end of the address. That means, communicate over port 8080.
So that's basically it. It might seem confusing but basically you should remember that communication must occur over a port, and TCP/IP data sent will have and IP address and a port number in the packet. If a computer isn't listening on the port your sending data over, then that data is ignored.
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