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Typically, peer-to-peer file-sharing programs, chat clients such as ICQ, Yahoo and MSN Messenger, and certain operations such as web serving and outgoing FTP require you to 'forward' or 'open' ports on your ADSL router to allow them to work.
Internet software applications interact with your computer through ‘ports' which need to be ‘open', or available, to that application. A port can be thought of as a part of the IP address which enables a device with a single IP address to handle several different processes simultaneously.
For example HTTP - which handles Web page requests - operates through Port 80. An ordinary web address can actually be entered specifying Port 80 as follows: http://www.netcomm.com.au:80 [The port does not usually need to be specified for Web page requests as the browser 'assumes' port 80. Note that if you tried it with :81 at the end it wouldn't normally work.]
Different types of application use different ports, for example: Telnet [port 23], FTP [20-21] SMTP [25]. There are 65,331 port numbers which can also be further specified as either TCP or UDP [however for the purposes of this article this distinction is not important].
THESE PORTS ARE VIRTUAL. The ports we are talking about opening are not physical ports anywhere on an actual switch. It is easy to believe that ‘ports' refer to an actual socket or plug, maybe at your ISP or somewhere else on the Internet. THEY DON'T.
Applications that need to interact with your computer will include a port number in their request for response – in other words, when they attempt to interact with your computer, their request will specify 'port number XXXX' as part of the routine. If the application can't locate the port, it will not execute. Result: you won't be able to chat, play the game, upload the file, or do whatever it is you are trying to do.
Furthermore, if you wish to host web or email services, then the appropriate ports must also be opened to be able to respond to requests from external Web Browsers and Mail Clients.
Why can't the application find the port? Because the default configuration of your router only deals with outgoing HTTP [=Web] and SMPT [=mail] requests and sorts incoming data packets according to which computer has requested them. Trying to run an application or serve web pages requires additional configuration. This is what you do when you forward or map a port.
When a port is forwarded, a table is created in the router which allows the request directed at port number XXXX to be 'forwarded' to a port attached to an 'internal IP address' found on one of the network's PCs, as specified by a port-forwarding rule.
The next page will provide illustrated examples of how this works.
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