Known applications (e.g. Telnet) are assigned fixed port addresses, which each application can refer to in the connection setup.
The range 0 ... 1023 contains reserved port addresses. Under no circumstances should these be used for your own applications. The complete list of reserved port addresses and their utilities are listed in RFC 1700 (1994).
Port addresses are also known as service connections, as they are used to call up a special application on the server or computer. A port address is set up by the network administrator.