Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 190
Assigned, Registered, and Dynamic Port Numbers

Reference RFC 1700 and the FTP site ftp://ftp.isi.edu/in-notes/iana/assignments (point your URL to this address on your browser). In the TCP/IP protocol, UDP port numbers come in three flavors: assigned, registered, and dynamic. Assigned numbers come in the range of 0–1023 and are fully controlled by the Internet Assigned Numbers Authority (refer to RFC 1700). These are numbers that deal with protocols such as TELNET, FTP, Network Time Protocol (NTP), and so on. No matter which implementation of TCP/IP (i.e., which vendor’s TCP) is in use, those applications listed beside the port number will always be the same (they are known as well-known port numbers). These are assigned by a central authority. In this case, RFC 1700 spells out which processes are assigned to which port numbers. Assigned port numbers (those formally reserved through Internet Address Numbers Authority) range from 0–1023 for TCP/UDP port numbers. After that, any application may use any port number beyond 1023 but less than 65,535. Some companies have registered their port numbers with IANA and other companies respect this by not using the same port number.

Here’s an example of an assigned port number: If station A wants to access the TFTP process on station B, it calls it in the UDP header with a destination port number of 69 (decimal). The source station requesting TFTP services also has a port number that is dynamically assigned by its TCP/IP stack. RFC 1700 suggests methods for assistance in assigning a dynamic port number. In this way, the server and client can communicate with one another using the port numbers to uniquely identify the service for that datagram.