CIDR / Subnet Calculator
Break down a CIDR block: network address, broadcast, usable IPs, subnet mask.
About Subnet Calculator
Breaks down a CIDR block into its network address, broadcast address, subnet mask, and the range and count of usable host addresses. Subnetting math is simple in principle but easy to get wrong under time pressure, and a single off-by-one error in a subnet plan can mean an unexpected address conflict discovered only after deployment. Works for both IPv4 and IPv6 CIDR notation, since IPv6 subnetting is easy to miscalculate by hand given the much larger address space.
Why usable host count is 2 less than the address count for IPv4: the first address is reserved as the network address, the last as broadcast — a /24 has 256 total addresses but only 254 usable hosts. IPv6 doesn't reserve a broadcast address the same way, a common point of confusion moving between the two.
Common Subnets
/31 → 2 hosts (p2p)
/30 → 2 usable
/29 → 6 usable
/28 → 14 usable
/27 → 30 usable
/26 → 62 usable
/25 → 126 usable
/24 → 254 usable
/23 → 510 usable
/22 → 1022 usable
/16 → 65534 usable
/8 → 16M usable
Related Tools
What CIDR notation means
In 192.168.10.0/24, the number after the slash is the prefix length: how many leading bits identify the network. The remaining bits identify hosts. An IPv4 address has 32 bits, so a /24 leaves 8 host bits — 28 = 256 addresses. Each step up in prefix length halves the block; each step down doubles it.
IPv4 subnet cheat sheet
| Prefix | Subnet mask | Addresses | Usable hosts |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /22 | 255.255.252.0 | 1,024 | 1,022 |
| /23 | 255.255.254.0 | 512 | 510 |
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
| /31 | 255.255.255.254 | 2 | 2 (point-to-point, RFC 3021) |
| /32 | 255.255.255.255 | 1 | 1 (single host) |
Subnetting tips
- Blocks must start on their own boundary. A /26 can start at .0, .64, .128 or .192 — never at .50. If the calculator reports a different network address than the one you typed, your input was not aligned.
- Plan from largest to smallest. Allocate the biggest subnets first so smaller ones fill the gaps without overlapping.
- Leave headroom. Moving a live network to a bigger prefix later means renumbering; picking one size larger at the start is usually cheaper.
- IPv6 is different. A LAN is normally a /64 regardless of host count, and there is no broadcast address.
Need to go the other way — from a start/end address pair to CIDR blocks? Use the IP range to CIDR converter.