As transport layer connections usually use ports for multiplexing/demultiplexing on both source and destination, every connection must also have a source port. You can see that using well defined ports is helpful here.įor the source port, it is a completely different story. You can still run http servers on different ports (say, 12345), but than, the user had to enter to reach the server. For your example, HTTP, that means, that if you start a request for, your browser (or other software) knows that http usually uses port 80, connects to that port to get the html page. The reasoning behind the well defined port numbers is, that services can only listen on specific ports, and if the numbers were known by convention, you had to memorize them along with the server adress. When that range became to full, there were also higher port numbers assigned. Usually, the ports were defined in the privileged low number range (up to 1024). Regarding the destination port, you nearly gave the answer yourself: For well defined application protocols, there were defined well known ports. The tcp protocol is a protocol of the underlying transport layer, so the definition / use of port 80 is not defined for tcp, but rather for the http application protocol (while the actual port usage takes place in the transport layer). In fact, there are two questions with two different answers.įirstly, you have to be exact: The port definition is part of the Internet Proctocol (commonly called TCP/IP) application layer.
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