Ports are a core feature of modern computing. I'm not talking about transferring versions of applications to another architecture or operating system. Instead, I mean the kind of port that is an address for a virtual connection point to or from a computer and another device or server, including the Internet. Ports direct external traffic to the correct application, and this function makes them important for troubleshooting and security. How do you find which ports are open or listening (i.e., currently in use) when there is no need for them to be? What ports are associated with which application or server? How do you know whether any ports are hidden and being used by an intruder? Even if you are working on a standalone computer, knowing how to answer these questions is a basic administrative skill.

Ports can be either hardware or software. Either way, they are treated similarly. Most ports are managed by two protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Both TCP and UDP have a range of port numbers, divided into three categories:

• System ports (numbers 0-1023): These are the most common ports and essential to external communication. Sometimes called well-known ports, system ports can only be used by root or privileged users. For example, ssh is port 22, and the Network Time Protocol (NTP) is port 123. While these ports can be changed, the changed ports can only communicate with other machines that have also reassigned their ports accordingly.
• Registered ports (numbers 1024-49151): These ports are available for processes and applications run by ordinary users. Registered ports are sometimes called user ports.
• Dynamic ports (numbers 49152-65535): These ports can be used by any processes or applications as needed, and they are assigned on the fly. Dynamic ports are also called private or ephemeral ports.

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