Free practice
Subnetting Practice
Subnetting is the one CCNA skill you are expected to do quickly and in your head. These two tools train the two halves of it: the arithmetic, and the design judgement it feeds.
/24 to /30, private ranges. The subnetting you meet most often.
How to subnet quickly
Under exam time pressure nobody converts to binary. The shortcut is to find the octet the mask ends in, work out the block size, and count. A /26 is 255.255.255.192, and 256 minus 192 is 64, so the subnets start at 0, 64, 128 and 192. Any host address tells you its subnet immediately: round down to the nearest multiple of the block size. The broadcast address is one below the start of the next subnet, and the usable range sits between the two. Every question in the drill above shows this working once you answer it.
Common questions
- How many usable hosts does a /26 have?
- 62. A /26 leaves 6 host bits, and 2^6 - 2 = 62. The two subtracted addresses are the network address and the broadcast address, which cannot be assigned to a host.
- Why do /31 and /32 not follow the 2^n - 2 rule?
- A /31 is the point-to-point case described in RFC 3021: on a link with only two devices there is no need for a network or broadcast address, so both addresses are usable. A /32 is a single host route, so it has exactly one address. These two are where most subnet calculators, and most exam candidates, go wrong.
- What is the block size or interesting octet method?
- Instead of working in binary, you find the octet the mask ends in and count in steps of 256 minus that octet's mask value. For a /26 the mask is 255.255.255.192, so the block size is 64 and subnets start at 0, 64, 128 and 192. It is far faster than converting to binary under exam time pressure.
- What is the difference between a subnet mask and a wildcard mask?
- They are inverses of each other. The subnet mask goes on an interface, while the wildcard mask is what IOS expects in ACL entries and OSPF network statements. A /24 is 255.255.255.0 as a subnet mask and 0.0.0.255 as a wildcard, and swapping the two is one of the most common configuration errors.
- Why does VLSM allocation order matter if the maths still adds up?
- Allocating the largest requirement first keeps the unused space in one contiguous block. Allocating small subnets first leaves gaps that are individually too small to hold anything you later need, so the block fragments and you run out of usable space long before you run out of addresses.
Take it to a real device
Getting the maths right is half of it. The other half is what a router does when you get it wrong, which is rarely an error message. The VLSM overlapping subnets lab puts a real overlap on a simulated router and asks you to find it, and the subnetting question bank covers the theory side.