Unlimited random subnetting problems with the working shown for every answer. Find network and broadcast addresses, host ranges, usable host counts, masks and the right prefix for a host requirement. Built for CCNA, Network+ and CCST Networking candidates.
Need the answer to one specific subnet? Use the subnet calculator or the subnetting cheat sheet.
Each round is 10 questions. Work them out on paper, then type the answer.
Every problem here can be solved by hand in four moves, and the working shown after each answer follows the same order.
Worked example: 172.20.77.130/19. A /19 mask is 255.255.224.0, so the interesting octet is the third and the block size is 256 - 224 = 32. The third octet is 77, and the largest multiple of 32 not above 77 is 64. Network = 172.20.64.0, broadcast = 172.20.95.255 (64 + 32 - 1 = 95), usable range 172.20.64.1 to 172.20.95.254, and 213 - 2 = 8,190 usable hosts.
For the theory behind it, read Subnetting explained and VLSM explained, then test yourself on exam-style questions.
Last updated October 30, 2026. Problems use private address space and prefixes from /9 to /30. Questions are generated randomly in your browser; they are practice drills written by CertSharp, not real exam questions.
Learn the block-size method, then repeat it on paper until it is automatic: find the interesting octet, work out the block size (256 minus the mask value), locate the network address, then derive broadcast and host range. Short daily rounds beat long occasional sessions. Start on Easy, move up once you score 9 out of 10, and time yourself on Hard.
Aim for under 45 seconds for a network, broadcast or host-range problem and about a minute for a prefix-for-hosts problem. The exam gives no calculator, and subnetting also feeds ACL wildcard masks, OSPF network statements and VLSM design questions, so speed on the basics saves time elsewhere.
Yes. Network+ expects you to work out subnets, masks, network and broadcast addresses and to choose appropriate address ranges. Cisco CCNA tests it heavily, including VLSM. CCST Networking covers the fundamentals at a lighter level. See the CertSharp CCNA and Network+ exam guides for current objectives.
Every answer and every worked step is calculated by the same code as the CertSharp subnet calculator, and the generator was checked against an independent calculation across tens of thousands of random problems. Treat /31 and /32 as outside this trainer: it stays between /9 and /30, where the standard network, broadcast and usable-host rules apply.
No account is needed and nothing is stored or sent anywhere: problems are generated in your browser and your score lives only on the page until you close it.
Try 30 free questions per session in practice mode for CCNA or Network+, or unlock the full 500-question bank and timed exam mode for $11.99 one-time per certification.