The two most popular Internet transport protocols, Transmission Control Protocol ( TCP) and the User Datagram Protocol ( UDP) and other less known protocols use port numbers for communication sessions (source and destination port numbers in conjunction with the source and destination IP addresses). It is a 16-bit number ( 0 to 65535) which differentiates one application from another on end systems. In computer networking, and more definitely in software terms, a port is a logical entity which acts as a endpoint of communication to identify a given application or process on an Linux operating system. In this article, we will briefly talk about ports in computer networking and move to how you can list all open ports in Linux.