The well-known port numbers are the port numbers that are reserved for assignment by the Internet Corporation for Assigned Names and Numbers (ICANN) for use by the application end points that communicate using the Internet's Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). Each kind of application has a designated (and thus "well-known") port number. For example, a remote job entry application has the port number of 5; the Hypertext Transfer Protocol (HTTP) application has the port number of 80; and the Post Office Protocol Version 3 (POP3) application, commonly used for e-mail delivery, has the port number of 110. When one application communicates with another application at another host computer on the Internet, it specifies that application in each data transmission by using its port number.
The well-known ports cover the range of possible port numbers from 0 through 1023. The registered ports are numbered from 1024 through 49151. The remaining ports, referred to as dynamic ports or private ports, are numbered from 49152 through 65535.
On most systems, a well-known port number can only be used by a system (root) process or by a program run by a privileged user. Before the arrival of ICANN, the well-known port numbers were administered by the Internet Internet Assigned Numbers Authority (IANA).