Subnetting Practice Drill — timed quiz with explanations (CCNA practice)
Practise subnetting against the clock: network and broadcast addresses, usable hosts and mask ⇄ prefix, with the block-size explanation after every answer, personal bests and a shareable challenge link with the same questions.
🔒 Runs entirely in your browser. Your input and output are never sent to a server or stored — fully private.
Shortcut: block size = 256 − the mask octet. Find the multiple of the block size at or below the address octet — that is the network; one less than the next multiple is the broadcast.
About this tool
This subnetting drill turns the most-tested skill in networking exams into a ten-question game against the clock. Pick a question type — network address, broadcast address, usable hosts, prefix to mask, mask to prefix, or a mix — and a difficulty from /24–/30 up to /8–/30, then type each answer. After every question you see the block-size working that gets you there, at the end you get a list of the ones to review, and your personal best is kept in this browser. Share your score to X, LINE, Facebook, Threads or Weibo: the link replays exactly the same questions, so friends and colleagues can try to beat you. Everything runs in your browser.
Frequently asked questions
What is the fastest way to subnet in your head?
Use the block size, sometimes called the magic number. Find the octet where the mask is neither 255 nor 0 and subtract it from 256: for a /27 the mask is 255.255.255.224, so the block is 32. Subnets start at multiples of 32 — 0, 32, 64, 96 — so 192.168.1.77 sits in the 64 block: network .64, broadcast .95 (one below the next multiple). With practice you skip binary entirely, which is exactly what this drill trains.
How many usable hosts does each prefix give?
2 to the power of the host bits, minus 2 for the network and broadcast addresses: /30 = 2, /29 = 6, /28 = 14, /27 = 30, /26 = 62, /25 = 126, /24 = 254, /23 = 510, /22 = 1,022. Two exceptions are worth knowing for exams: a /31 point-to-point link uses both of its addresses (RFC 3021), and a /32 is a single host route or loopback. The drill sticks to /8–/30, where the minus-2 rule always applies.
Why are /16–/23 questions harder?
Because the interesting octet moves to the third one, and the fourth octet stops being part of the calculation. For 172.20.77.130/20 the block is 16 in the third octet, so 77 falls in 64–79: the network is 172.20.64.0 and the broadcast 172.20.79.255 — the last octet is simply 0 or 255. Most mistakes at this level are applying the block to the wrong octet or forgetting to reset the octets after it, which the explanations point out.
Which exams test subnetting, and how fast should I be?
Cisco CCNA, CompTIA Network+ and, in Japan, the IPA exams — Fundamental and Applied Information Technology Engineer and Network Specialist — all include it, and in practice it is a daily task for network engineers. Exams are timed, so accuracy alone is not enough: aim to answer a mixed normal-level question in under 20 seconds with no paper. Use Mixed mode for exam practice and a single type to drill a weak spot.
How do challenge links work — is my score uploaded?
Nothing is uploaded. The questions come from a random seed, and the share link simply carries that seed, the mode, the difficulty and your score in the URL. Whoever opens it gets the same ten questions and sees the score to beat; there is no server, account or leaderboard. Your personal best is stored only in this browser's local storage, per question type and difficulty, and disappears if you clear site data.