IP Subnet Calculator - CIDR, Mask & Host Range

An IPv4 address is 32 bits. The CIDR prefix says how many of those bits, counting from the left, belong to the network; the rest identify the host. A /24 reserves the first 24 bits for the network and leaves 8 for hosts, which is why a /24 holds 2⁸ = 256 addresses. Move the line one bit right to /25 and you halve the block to 128 addresses; move it left to /23and you double it to 512. Every question this tool answers — network address, broadcast, host range, mask — comes from where that single line sits.

The subnet mask is the same line expressed as an address: 24 leading one-bits followed by 8 zero-bits is 11111111.11111111.11111111.00000000, which reads as 255.255.255.0. That is all a mask is — the network/host boundary written in dotted-decimal so a router can apply it with a single bitwise AND. This is also why masks only ever contain the values 0, 128, 192, 224, 240, 248, 252, 254, and 255: those are the only bytes you get from a run of ones followed by a run of zeros.

This is the single most common surprise. Enter 192.168.1.100 with a /27 and the network address comes back as 192.168.1.96, not 192.168.1.100. Nothing is wrong. A /27 leaves 5 host bits, so subnets step every 2⁵ = 32 addresses: the boundaries are .0, .32, .64, .96, .128, and so on. The address .100 lives inside the block that starts at .96, so that is its network address, and the broadcast is .127— the last address before the next block begins at .128.

The tool does this snapping on purpose. In practice you rarely know a subnet’s network address off the top of your head, but you always know an address some device is using — a router interface, a printer, a DHCP lease. Paste that in and the calculator tells you which subnet it belongs to and what the usable range around it is. Feeding it the exact network ID is optional, not required.

It calculates a single IPv4 subnet from one prefix. It does not do variable-length subnet masking (VLSM) across a plan — carving a /24 into a mix of /26s and /28s for departments of different sizes is a design exercise you drive one block at a time. It does not calculate IPv6 subnets, which follow different rules with a 128-bit address and no broadcast concept at all. And it does not check whether your chosen block overlaps something already in use — overlapping CIDR ranges between on-premises networks, VPN gateways, and cloud VPCs are a real source of routing failures, but detecting them needs the whole address plan, not one subnet.

For the one job it does, the arithmetic is exact and deterministic: bitwise masking on a 32-bit integer, computed in your browser, with the full /0–/32range handled without the integer-overflow errors that plague naive implementations. Use it to check a boundary, size a block, or settle a “which subnet is this address in” question in seconds — then verify against your actual routing tables before you deploy.