Unlimited random IPv4 subnetting problems, each with a step-by-step worked solution. Answer, build a streak, and check your working. Nothing is sent anywhere — it all runs in your browser.
Every problem here gives you an address and a prefix, like 192.168.23.138/21, and asks for one of three things:
/21 is 255.255.248.0), line it up under the IP in binary, and AND the two together. Every host bit becomes 0./21 leaves 32 − 21 = 11 host bits, so 211 = 2048 total addresses. Subtract two — one for the network address, one for the broadcast — to get 2046 usable.Prefixes here run from /8 to /30. If you get stuck, hit Show worked solution to see the binary lined up.
Subnetting is on the CCNA and CompTIA Network+ exams without a calculator, and it comes up constantly in real network design and troubleshooting — working out whether two hosts are on the same subnet, how many addresses a VLAN needs, or which range a firewall rule should cover. Doing it on paper until it is fast is the point.
Need to check a full breakdown of any block? Use the Subnet Calculator. New to the topic? Start with Understanding Subnets and IP Addresses or What is CIDR.
No. Each problem is generated fresh from a random address and a random prefix between /8 and /30, so you effectively never see the same one twice. There is no fixed question bank.
Memorise the mask value for each prefix (/24 is 255.255.255.0, /26 is 255.255.255.192, and so on), and only convert the one octet where the network boundary falls to binary. Use the hint for the method and the worked solution to check your working, and drill until a problem takes under 30 seconds without a calculator.
Not yet. Tier 1 covers core IPv4: network and broadcast addresses, subnet and wildcard masks, host counts and ranges, and CIDR from a mask. VLSM, route summarisation and IPv6 are planned for later tiers.