Friday, October 16, 2009

Ports are not only for sailors

When dealing with TCP or UDP packets, you often hear the term port. No, this doesn’t mean that these protocols are designed for mariners. Instead, a port is a number that identifies where a packet came from on the sending host, and where it should go to on the receiving host. If you compare an IP address to a street address for mail delivery, the port number is like an apartment number.
When a server application, such as a Web server, runs on a computer that uses TCP/IP, it reserves a port on that computer. This reservation is nothing more than telling the networking software that any packet that is addressed to this port should be forwarded to the server application. Any application that sends TCP or UDP packets also sends them from a port. This way, the TCP/IP stack knows what application should receive return packets. In addition to source and destination addresses, IP packets also contain source and destination ports.

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