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World L3: Phone Lines

Ports and listening: ss

You will learn: what a port is, and how to see who answers each one.

Checked with Linux (Debian 13) on 1 Nov 2026

The problem

You are hereyou@kedai:~$

Folder ~

Empty folder

+ 3 hidden (ls -a)

Services

  • cronrunningstarts at boot
  • sshrunningstarts at boot
  • nginxrunningstarts at boot

Phone lines (network) Address 10.0.0.5

Listening ports

  • :22 sshd
  • :80 nginx

A customer dials kedai, extension 8080. The real error says Could not connect to server. Plain words: nobody waits at that extension.

Try it

The IP address reaches the building. The port: An extension number on the shop's phone line. Web is 80, HTTPS 443, SSH 22. picks the extension inside. Some are famous. Match each job to its port.

Drag a card into a box. Or tap the card, then tap the box.

        Watch

        A program waiting by an extension is listening. ss -tln lists them: TCP, listening, numbers. Tap the line for the web counter.

        Tap the line for port 80.

        0 of 1 found

        Try it

        Who answers each port? Add -p for the program names. Only root may see them, so use sudo.

        You are hereyou@kedai:~$

        Folder ~

        Empty folder

        + 3 hidden (ls -a)

        Services

        • cronrunningstarts at boot
        • sshrunningstarts at boot
        • nginxrunningstarts at boot

        Phone lines (network) Address 10.0.0.5

        Listening ports

        • :22 sshd
        • :80 nginx

        Predict

        You stop nginx. Then you run curl localhost. What happens?

        Try it

        Try it. Stop nginx, look with ss -tln, then start it again.

        You are hereyou@kedai:~$

        Folder ~

        Empty folder

        + 3 hidden (ls -a)

        Services

        • cronrunningstarts at boot
        • sshrunningstarts at boot
        • nginxrunningstarts at boot

        Phone lines (network) Address 10.0.0.5

        Listening ports

        • :22 sshd
        • :80 nginx

        Check

        True or not?

        Swipe right for good, left for needs work. Or use the buttons.

        1 / 3

        One address can have many ports listening.

        You did it!

        You can now see which ports listen, and who answers them.

        Go deeper (optional)

        ss -tln letters: t TCP, l listening, n numbers, p programs. Add u for UDP. Old guides use netstat; Debian 13 doesn’t install it. /etc/services lists famous port names.

        Real mission

        1. sudo ss -tlnp

        0 of 2 steps done · Your ticks are saved.