Glossary
Port
A number identifying a specific service on a machine.
An IP address identifies a machine; a port identifies which service on it. Web servers listen on 80 and 443, SSH on 22, PostgreSQL on 5432. Ports below 1024 require root privileges to bind.
Two everyday questions follow: "what is listening here?" (ss -tlnp) and "why is this port taken?" — usually an orphaned previous instance of your own app. The listen address matters too: 0.0.0.0 is reachable from the network, 127.0.0.1 only locally.
See it in practice
$ sudo ss -tlnp | grep :8080Names whatever holds the port, with its PID — the answer to "address already in use" in one line.
Where you'll meet it
See listening ports, active connections and which process owns each socket.
killTerminate processes politely or forcibly — and understand what -9 really does.
sshLog into remote machines, run remote commands, use keys instead of passwords, and tunnel ports.