Subnet Mask Calculator

The subnet mask decides how many addresses a network holds. This calculator answers the questions that come up when planning one: which mask fits 50 hosts, how small subnets get when a /24 is cut into five, what /20 looks like in dotted form, and which prefix a mask or wildcard mask such as 0.0.15.255 stands for. A full /0–/32 table is included for quick reference.

How to use this tool

1. Pick what you know: hosts needed, subnets needed, a prefix, a dotted mask or a wildcard mask.
2. Enter that value in its field (the other fields are ignored).
3. Read the prefix, dotted mask, wildcard mask and host count.
4. Use the reference table to compare neighbouring sizes.

Formula

usable hosts = 2^(32 − p) − 2 · smallest p with 2^(32 − p) − 2 ≥ hosts · new p = base p + ⌈log₂ subnets⌉

p is the prefix length (the number of 1-bits in the mask). A valid mask has all its 1-bits on the left; the wildcard mask is the same mask with every bit inverted, as used in Cisco ACLs and OSPF.

Example

You need 50 hosts on a LAN. A /27 holds 32 − 2 = 30 hosts, too few; a /26 holds 64 − 2 = 62, so the mask is 255.255.255.192 (/26) with wildcard 0.0.0.63, leaving 12 spare addresses. Dividing a /24 into 5 subnets needs 3 extra bits (2³ = 8 ≥ 5), giving /27 subnets of 30 hosts each.