How to calculate a CIDR prefix from the required host count

Short answer

To select an IPv4 prefix from a host count, find the smallest number of host bits h for which 2^h - 2 ≥ required hosts in a traditional subnet. Then calculate the prefix as 32 - h. Fifty hosts need six bits because 2^6 - 2 = 62, so the corresponding prefix is /26.

Worked example for 50 hosts

Try the smallest candidate sizes first:

5 host bits → 2⁵ = 32 addresses → 30 usable hosts
6 host bits → 2⁶ = 64 addresses → 62 usable hosts

Five bits are insufficient and six are enough. The prefix is therefore:

32 - 6 = /26

Its dotted-decimal mask is 255.255.255.192. Enter /26 in the IPv4 subnet calculator to confirm 64 total addresses and 62 traditionally usable addresses.

Quick capacity table

The table is not a complete network plan. Reserve room for future devices, infrastructure interfaces, and design changes before assigning the final block.

Why two addresses are subtracted

In a conventional IPv4 subnet, the first address identifies the network and the last is the directed broadcast address. Usable capacity is consequently calculated as 2^h - 2.

There are exceptions. On a /31 point-to-point link, both addresses can name the endpoints and no dedicated broadcast address exists. A /32 identifies a single host route. Do not apply the “minus two” formula to those prefixes.

From capacity to an actual block

Choosing /26 does not make every address a valid network address. Within a /24, /26 networks advance in blocks of 64:

192.168.10.0/26
192.168.10.64/26
192.168.10.128/26
192.168.10.192/26

192.168.10.50/26 is a host inside the first network, not the network address. The calculator normalizes the input and shows the correct boundary.

Frequent mistakes

• Choosing the first block whose total exceeds the host count without reserving network and broadcast addresses.
• Subtracting the reserved addresses twice.
• Confusing the number of hosts with the number of subnets.
• Writing a host address as though it were the network address.
• Applying the traditional formula to /31 and /32.
• Sizing exactly to today’s limit without operational headroom.

RFC 4632 describes CIDR prefix lengths and power-of-two block sizes. Calculate capacity first, verify block alignment next, and finally document the reservations used by the actual network.