In a server, more than one user process can use TCP at the same time. To identify the data associated with each process, port numbers are used. Port numbers are 16-bit, and numbers up to 65535 are possible, although in practice only a small subset of these numbers is commonly used.
|Service||Well-known port number|
|File Transfer Protocol (FTP)||21|
|Hypertext Transfer Protocol (HTTP)||80|
|HTTP with Secure Sockets Layer (SSL)||443|
Well-known ports are used only to establish communication between client and server processes. After that, the server allocates an ephemeral port number for subsequent use. Ephemeral port numbers are unique port numbers, which are assigned dynamically when processes start communicating. They are released when communication is complete.