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World L3: Phone Lines

Subnets and CIDR

You will learn: what /24 means, and when a call goes through the gateway.

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

kedai is 10.0.0.5/24. What does the /24 mean? It decides who kedai can call directly.

Try it

Think of a street. The first three numbers name the street. The last number is the house. That street is a subnet: One street of addresses. Machines on the same street call each other directly.. Which houses are on kedai’s street?

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

      Watch

      The slash number is CIDR. It counts how much of the address names the street. A bigger slash number means a shorter street. Put them from fewest houses to most.

      Fewest houses at the top. Put these in order. Use the arrows, or drag.

      • /28: 16 houses
      • /16: 65,536 houses
      • /32: one house
      • /24: 256 houses

      Check

      A /24 street has 256 numbers. Two are kept: .0 names the street and .255 calls every house at once. How many computers fit?

      Try it

      Calls to other streets go to the gateway: The telephone exchange at the end of the street. Calls to other streets go through it.: the telephone exchange. Find its number with ip route.

      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

      kedai calls 10.0.0.20. Does that call go through the exchange at 10.0.0.1?

      Check

      True or not? Swipe each one.

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

      1 / 3

      10.0.0.77 is on the street 10.0.0.0/24.

      You did it!

      You can now read /24 and tell when a call goes through the gateway.

      Go deeper (optional)

      Under the hood, /24 means the first 24 bits of 32. The old way to write it is a mask: 255.255.255.0. Cloud networks use the same idea. In World A3, you plan subnets for a mall.

      Real rep (optional):

      Real mission

      1. ip route

      0 of 2 steps done · Your ticks are saved.