Free VLSM Calculator
Variable Length Subnet Masking (VLSM) lets one network serve subnets of different sizes without wasting addresses. Enter your base network, list how many hosts each subnet needs, and this calculator allocates the smallest block that fits each one — largest first, which is exactly the method CCNA and Network+ VLSM questions expect. New to subnetting? Start with the subnet calculator and our guide How to Subnet IPv4 in Under a Minute.
| Subnet | Network / CIDR | Mask | Usable range | Usable hosts | Broadcast |
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Why allocate largest first?
Subnet blocks must start on a boundary that is a multiple of their size — a /26 can only begin at .0, .64, .128 or .192. If you place a small subnet first, the next big block may no longer fit on a clean boundary and addresses get stranded. Sorting from the largest requirement to the smallest guarantees every block lands on a valid boundary with no gaps, so the plan always fits if it can fit at all. That is the exam technique too: biggest subnet first, then work down.
VLSM is guaranteed CCNA territory — practice free
VLSM and subnetting questions appear on every CCNA 200-301 and Network+ N10-009 exam. Try the free 30-question samples and check your speed.
Free CCNA SampleFree Network+ SampleFAQ
What is VLSM?
VLSM (Variable Length Subnet Masking) means using different subnet sizes inside one network instead of cutting it into equal pieces. A department with 50 hosts gets a /26, a point-to-point link gets a /30, and no addresses are wasted on one-size-fits-all subnets.
How does this VLSM calculator allocate subnets?
It sorts your requirements from largest to smallest, then gives each subnet the smallest power-of-two block that fits its hosts plus the network and broadcast addresses, placed on a valid block boundary. Largest-first allocation is the standard exam method because it never strands unusable gaps.
How many hosts fit in a /26, /27, /28 and /30?
A /26 holds 62 usable hosts, a /27 holds 30, a /28 holds 14 and a /30 holds 2. Usable hosts = block size − 2, because the first address is the network address and the last is the broadcast address.
What happens if my subnets do not fit in the base network?
The calculator tells you instead of producing a wrong plan: the requirements need more addresses than the base network has. Use a larger base network (a smaller prefix, such as /23 instead of /24) or reduce the host counts.