TCP Ports and Sockets

Three identification numbers are commonly used when referring to TCP sockets.

• The IP address identifies the computer on the network.
• The TCP port number identifies a process or application inside the computer.
• The socket identifies both the computer and the process simultaneously. A socket is used as an endpoint.

Note

TCP ports are known by a variety of names. These names include TCP port number, TCP port address, TCP port, port number, port address, port, and data port.

Any application or process that uses TCP for its transport is assigned a unique identification number called a TCP port. TCP ports specify the path of communication between client and server applications. These ports are numbered beginning with zero. Port numbers for client applications are dynamically assigned by the operating system when there is a request for service. Port numbers for server applications are preassigned by the Internet Assigned Numbers Authority (IANA) and do not change.

IANA is the group that assigns processes to port numbers 0 through 1023. This range of numbers is reserved for services. A client application or process that uses TCP as a transport is assigned a port number greater than 1023 by the operating system.

A server application or process that uses TCP as a transport has at least one preassigned port number. For example, the preassigned port numbers for FTP server services are 20 (data) and 21 (control). These port assignments are called the "Well Known Port Numbers" and are documented in RFC 1700 (see http://ds.internic.net/std/std2.txt). Table 1.7 is a short list of some Well Known Port Numbers. (For more port numbers, see Appendix B, "Port Reference for Microsoft TCP/IP," in the Windows NT Server Networking Guide.)

Table 1.7 Some Well Known Port Numbers

Port numbers are used in conjunction with an IP address to form a socket. Sockets always have a number (or address) associated with them and designate an endpoint. Examples of socket numbers are shown in Table 1.8.

Table 1.8 Socket Examples