Ports
When a virtual connection is set up between two communicating systems, each end is tied to a port. The port is an identifier used by the TCP software rather than an actual physical device, and it allows multiple network connections to be made on one machine by different applications.
When a message is received by the TCP software running on a host computer, the data is sent to the correct application based on the port number. By convention, a well-known port is normally used by a server providing a well-known service. A list of well-known ports for various applications is maintained by Internet Assigned Number Authority (IANA) and can be found at http://www.isi.edu/in-notes/iana/assignments/port-numbers. For example, the File Transfer Protocol (FTP) uses port 21, and a web server uses port 80.
Systems with TCP/IP software installed have a services file that lists the ports used on that machine. This file is often preconfigured for well-known applications and is maintained by the system administrator to reflect the actual port usage on the machine. This file is usually /etc/services on a Linux system.