Port Number

In TCP (Transmission Control Protocol) and UDP (User Datagram Protocol), a port number is a 16-bit integer placed in the header of a message. It is logically passed between client and server transport layers. A port number is physically passed between the transport layer and Internet Protocol layer and forwarded to the destination. For instance, a client request to an internet server may be a request for a file served by the host’s FTP (File Transfer Protocol) program. In order for the request to be passed to the remote server’s FTP process, the TCP program layer in the client computer must identify a port number that is appended to that particular request. The TCP layer reads the port number at the server and forwards the client request to the FTP program. Permanent port numbers are assigned in a few processes and services. These are called well-known port numbers. In other instances, a port number may be temporarily assigned for the duration and completion of a request.