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IP Ports

While an IP address is like a telephone number, an IP port is like a telephone extension number. For offices with one telephone number, they can have more than one telephone conversation simultaneously. Extension of one office calling the extension of another office.

On a computer, you can have more than one conversation with other computers. For example, you can have IE and Firefox running simultaneously both refering to yahoo.com. You can even have two Firefox tabs running. How does yahoo.com know which browser or tab it needs to send a response?

Each communication session is allocated an IP port by its computer host.

The browser normally operates on hypertext transfer protocol (HTTP). The default port for HTTP is 80 and for secure (https) mode, it is port 443. Therefore, when you key in http://www.yahoo.com, the browser would actually address it as http://www.yahoo.com:80.

http://www.yahoo.com:80 means that you are requesting information from the computer with the address www.yahoo.com at port 80.

But the telephone extension you are calling needs to know your extension too, so that the telephone network knows where to send the voice who is responding to your call. When a browser request calls www.yahoo.com:80, the browser request obtains an unused port number from its computer so that www.yahoo.com:80 could respond to the browser request through that port. Just as you would go to the nearest unused telephone extension to make a call.

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