Most server-based IP applications use what are referred to as "well-known" port numbers. For example, an HTTP server will listen on TCP port 80 by default, which is the "well-known" port number for HTTP. Since there is no "registry" of port numbers that defines which port number an application is listening on, server-based applications need to listen for client connection requests in a consistent manner. They achieve this by using the same port number all the time.
Well-known services like HTTP and SMTP mail have reserved port numbers, so that they can always use the same ones. This allows web browsers to always go to the same port number, and also allows e-mail clients to always send messages to the same port number, regardless of the server that these applications are running on. All port numbers below 256 are reserved for well- known applications, although not all of them have yet been assigned. Any server service can run on ports between 256 and 1024, which are considered to be privileged by the TCP/IP stack. These ports will always be used to listen for incoming connection requests, and any incoming data will be passed off to any application that is listening on that port number.
For more information on ports and sockets, refer to section B.5.1 Port Numbers and Sockets. For more information on the application clients, refer to section B.5.3 Clients Initiate the Connection. For more information on how clients and servers communicate with each other, refer to section B.5.4 Application Linguistics.
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