VLSM Calculator

VLSM allows a network administrator to divide an IP address space into subnets of different sizes, each with its own prefix length. Unlike fixed-length subnetting (FLSM), which forces all subnets to be the same size, VLSM lets you allocate exactly as many addresses as each subnet needs — a /30 for a 2-host point-to-point link, a /25 for a 126-host LAN, and anything in between, all carved out of the same parent block.

The standard VLSM algorithm sorts subnets from largest to smallest host requirement. For each subnet it finds the smallest power-of-2 block that fits (required hosts + 2 for the network and broadcast addresses). Blocks are then assigned sequentially from the start of the parent network, aligned to their natural boundaries. Allocating largest-first avoids fragmentation and guarantees the most efficient packing possible within a given address range.

Use VLSM whenever your subnets have significantly different size requirements — for example, a /24 LAN, a /27 server VLAN, and several /30 point-to-point WAN links all carved from a single /20. Fixed-length subnetting (FLSM) wastes addresses because every subnet gets padded to the same size. VLSM is the industry standard for modern enterprise and ISP network design because it makes optimal use of scarce address space.

Every IPv4 subnet reserves two addresses: the network address (the first address, all host bits zero) and the broadcast address (the last address, all host bits one). Every address between them is usable by hosts. A /24 block has 256 total addresses — 1 network + 1 broadcast + 254 usable. A /30 has 4 total — 1 + 1 + 2 usable, which is the classic point-to-point link subnet. A /31 (RFC 3021) omits broadcast and gives exactly 2 usable addresses for router-to-router links.

They represent the same information in two different formats. A CIDR prefix like /24 is simply the count of consecutive 1-bits in the subnet mask. The /24 prefix corresponds to the subnet mask 255.255.255.0 (24 ones followed by 8 zeros in binary). CIDR notation is more compact and easier to read; subnet masks in dotted-decimal form are what network devices historically accepted in configuration. Modern equipment accepts both, and this calculator shows you both for every subnet it allocates.