Cisco Field Manual[c] Router Configuration

Appendix G. Well-Known IP Port Numbers

Transport layer protocols identify higher-layer traffic with 16-bit fields called port numbers. A connection between two devices uses a source and a destination port, both contained within the protocol data unit. Figure G-1 shows the User Datagram Protocol (UDP) header format, with the source and destination port fields shaded. Figure G-2 shows the Transmission Control Protocol (TCP) header format, with the source and destination port fields shaded.

Both UDP and TCP port numbers are divided into the following ranges:

• Well-known port numbers (0 through 1023)

• Registered port numbers (1024 through 49151)

• Dynamic or Private port numbers (49152 through 65535)

Usually, a port assignment uses a common port number for both UDP and TCP. A connection from a client to a server uses the well-known port on the server as a service contact port, while the client is free to dynamically assign its own port number. For TCP, the connection is identified by the source and destination IP addresses, as well as the source and destination TCP port numbers.

Well-known or assigned IP protocols are registered with the Internet Assigned Numbers Authority (IANA). The information presented here is reproduced with permission from the IANA. For the most current IP protocol number assignment information, refer to http://www.iana.org/numbers.htm under the "Port Numbers" link.

Table G-1 shows some commonly used protocols, their port numbers, and a brief description. The IANA has recorded close to 3350 unique port numbers. Due to space limitations, only a small subset of these have been presented here.

Table G-1. Commonly Used Protocols and Associated Port Numbers