TCP
Where IPv4 and IPv6 merely specify how packets are sent across a Network, TCP adds the concepts of “Connections” and TCP Ports (among others).
A TCP Port provides multiplexing, allowing multiple networking applications to run on a single host by assigning TCP Port numbers to the various applications. A TCP Port can range anywhere between 0 and 65535. Quite a few port numbers have been reserved, or have a predefined application. I.e.: port 80 is reserved for applications that talk HTTP, and port 25 is reserved for SMTP.