VLSM Subnet Calculator

How to use

1. 1Enter the parent block you are dividing in CIDR notation, for example 172.16.0.0/22, or load one of the quick samples to see a full plan.
2. 2List each subnet you need with the number of hosts it must support. Use 2 for a point-to-point link and 1 for a single host route.
3. 3Add or remove rows as needed. Empty rows are ignored, so you can leave spare rows in place while you plan.
4. 4Read the allocation: each subnet is shown largest first with its CIDR, mask, network, usable range, broadcast, and wildcard, packed in order from the start of the parent block.
5. 5Check the utilization panel for how much of the block is used and what free space is left, then copy the full plan as text for your documentation.

About this tool

VLSM Subnet Calculator turns a single IPv4 block into a complete address plan. Variable Length Subnet Masking is the practice of carving one parent network into subnets of different sizes so each one is just big enough for the hosts it carries, instead of splitting a block into equal pieces and wasting the space that fixed-length subnetting leaves behind. You give the tool a parent block in CIDR notation, such as 172.16.0.0/22, and a list of the subnets you need with the number of hosts each must support, and it works out the rest. The defining step is the order of allocation. Each requirement is first rounded up to the smallest power-of-two block that holds its hosts plus the two addresses every normal subnet reserves for the network and broadcast, so a request for 500 hosts becomes a /23 with 510 usable, a request for 200 becomes a /24 with 254 usable, and a request for 60 becomes a /26 with 62 usable. The blocks are then sorted from largest to smallest and packed contiguously from the start of the parent, which is what makes a VLSM plan valid: a block of a given size always lands on a boundary that is a multiple of its own size, so every subnet is correctly aligned and there are no gaps left by mismatched alignment. Two host counts are handled specially because real networks use them constantly. A request for two hosts produces a /31 point-to-point link under RFC 3021, where both addresses are usable and there is no broadcast, the modern way to number a router-to-router link without burning a /30. A request for a single host produces a /32 host route. For every allocated subnet the tool reports the CIDR, the dotted-decimal subnet mask, the network address, the usable host range, the broadcast address, the wildcard mask used in router ACLs, the number of usable hosts, and how many spare host addresses are left over so you can see at a glance whether a subnet is a tight fit or has room to grow. A utilization panel sums it all up: how many of the parent block's addresses the plan consumes, the percentage used, and the exact range of any free space that remains, tagged with the largest aligned block that would fit there if you need to hand it out later. If the requirements together ask for more addresses than the parent block can hold, the tool does not silently overflow; it reports the shortfall and the total each side needs so you can choose a larger supernet or trim a requirement. This is the calculation network engineers do when sizing VLANs and DHCP scopes, that students work through for the CCNA and CompTIA Network+ exams, and that anyone designing an address scheme reaches for when a flat equal-split would waste half the block. It is deliberately different from a plain subnet calculator, which describes one subnet you already have. This one designs many subnets you do not have yet from a parent block and a set of host counts. Everything runs in your browser on plain integer math. The blocks, names, and host counts you enter, including production network plans, never leave your device and no lookups are made.

Free to use. Works in your browser. No signup, no login.