Subnetting practice
Subnetting is the one CCNA skill you have to do by hand, under time pressure, with no calculator. So drill it: this tool throws random IPv4 questions at you and grades every field the instant you check — mask, network, broadcast, host range, and host count. Unlimited reps, right in your browser.
How to work out a subnet by hand
Turn the prefix into a mask
The prefix (like /26) is how many bits are network. Fill that many 1s from the left: /26 is 11111111.11111111.11111111.11000000 = 255.255.255.192. The last non-255 octet is the “interesting” octet.
Find the block size
Block size = 256 − (the interesting octet’s mask value). For /26 that’s 256 − 192 = 64. Subnets start at multiples of the block size (0, 64, 128, 192) in the interesting octet.
Locate the network and broadcast
The network address is the multiple of the block size at or just below the host’s interesting octet. The broadcast is the next multiple minus one. Everything between them (exclusive) is the usable host range.
Count the hosts
Usable hosts = 2^(host bits) − 2. A /26 has 6 host bits, so 2^6 − 2 = 62 usable hosts. (The −2 is the network and broadcast addresses — except a /31, which has 2 usable per RFC 3021.)
Want to check your work? Use the subnet calculator. For the full method with worked VLSM examples, read How to Subnet with VLSM or browse the Cisco guides.
Subnetting practice — common questions
How do I practice subnetting for the CCNA?
Do lots of timed reps. The CCNA gives you no calculator, so the skill is finding a subnet's mask, network, broadcast, host range, and host count by hand, fast. This free tool generates unlimited random questions and grades every field instantly, so you can drill until it's automatic.
Is this a subnetting calculator?
It's the opposite of a calculator — you enter the answers and it checks them. A calculator does the math for you; this makes you do it and tells you what you got wrong, which is how you actually get faster for the exam. Use a calculator to verify, this to learn.
What does each answer field mean?
Subnet mask is the dotted-decimal mask for the given prefix (a /26 is 255.255.255.192). Network address is the first address in the subnet; broadcast is the last. First and last usable host are everything in between. Usable hosts is 2^(host bits) − 2 (subtracting the network and broadcast addresses).
How fast should I be able to subnet?
Most people aim to answer a full subnetting question in under a minute, and the fastest do it in seconds. Start on the Class C (/24–/30) level to build the pattern, then move to Mixed and All as it clicks.
More free tools
Subnet Calculator →
Enter an IPv4 address and prefix — get the network, broadcast, host range, masks, and host count instantly.
VLSM Calculator →
Allocate variable-length subnets from a base network by per-subnet host requirement, largest-first.
Well-Known Ports →
A searchable TCP/UDP port reference — SSH 22, DNS 53, HTTPS 443 — plus a quiz to drill them.
OSI Model Quiz →
A protocol or device appears; you pick the right OSI layer. Instant feedback and a streak.
Try the Grader →
Edit a real Cisco IOS config in your browser and get instant per-requirement pass/fail.
Subnetting is one skill. The exam tests dozens.
Practice the rest the same way — build a real Cisco lab and grade your own config. Start with the free sample.