Q & A on Ports and Sockets Q: 1. can you clarify the difference between a port and a socket? A: A socket is an operating system abstraction similar to a file descriptor; it is part of the Application Program Interface (API). A program creates a socket, specifies that it will be used with TCP/IP, and then fills in details such as whether the socket will be used by a client or a server. A port is a transport-layer abstraction that is part of the TCP/IP suite. Each port is a 16-bit integer; the space of ports for TCP and UDP are separate. Ports used by servers are given reserved values (e.g., a Web server uses port number 80). Note that after creating a socket, a program specifies a port to be used with that socket. Q: 2. also when can two processes end up sharing the same port (except the case when forking is done after binding to a port then parent-child share it). Yes, two processes can share the same port. Sharing is most common in TCP because TCP identifies a connection by 4 items: Client IP address Client port number Server IP address Server port number Thus, two processes can provide Web service on port 80 concurrently as long as the two TCP connections go to different clients.