TCP uses numbered ports in its operations. The ports can either function as origins of connections or recipients. The numbers allocated to the ports are called well known numbers. The clients make their connection from a high port. While most of the numbers fall below 1024, the high ports are numbered above 1024. The ports with numbers below 1024 were traditionally called trusted ports. Ports with numbers that fall below 1024 can only be accessed by privileged people on the UNIX machines.
Part of the duties of Internet Assigned Numbers Authority is the assignment of port numbers to ports on the Operational System Interconnection (OSI) model. Examples are number 80 that is assigned to HTTP Protocol that is used in the World Wide Web, 443 that is assigned to HTTPS Protocol and port 23 that is allocated to telnet. Port 25 is allocated to SMTP and the list goes on and on. Protocols can be run theoretically on any of the many ports. What IANA does is assign numbers that are only proposals to the world as to the things to expect on various ports. The suggestions are applicable whether the port is a mail server, FTP, e-mail or any other protocol.
For a complete list of port numbers and their duties, refer to the IANA website at www.iana.org