Learn subnetting

Lessons, practice with worked solutions, speed drills, exams and VLSM design — free, in your browser.

Subnetting is the one CCNA skill that is pure arithmetic, so it is taught here as a method you can run in your head, then drilled until it is fast. Sign in with a free account to start.

What is inside

• Lessons — binary and address basics; masks, prefixes and wildcards; network, broadcast and hosts; counting subnets and hosts; subnet design and summarisation; IPv6
• Practice — 18 kinds of question at three levels, with the whole working shown when you are wrong
• Speed drill — sixty seconds, as many as you can
• Exam mode — twenty questions, twenty minutes, marked at the end
• VLSM challenge — design a network from requirements; any correct design passes
• Cheat sheet — every prefix from /8 to /32: mask, block size, hosts
• Calculator — subnet, split, summarise, IPv6 and binary

The method

1. Find the interesting octet: where the mask is not 255 or 0.
2. Block size = 256 − that mask octet.
3. Count up in blocks to find the one your address falls in.
4. Network = start of the block; broadcast = start of the next block − 1.
5. Usable hosts = 2^{host bits} − 2.

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