Networking Spot it

Spot the mistake: choosing a prefix

Can you spot the mistake before the answer turns red?

Lesson notes

Key points

We need at least 62 usable addresses, so 2^h minus 2 must be 62 or more.
Try h = 6: 2^6 is 64, so 6 host bits are enough.
Host bits are 6, so the prefix is 32 minus 6, which is /26.
A /26 gives 62 usable hosts, exactly enough for the lab.

Takeaway: Always subtract 2 before checking that host bits are enough.

Quick check: Which step is wrong?

Step 1
Step 2
Step 3

64 minus 2 is 62, which fits, but step 2 forgot to subtract the network and broadcast.

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