IP & Domain Tools

Subnet Mask to CIDR Prefix Calculator

Enter a subnet mask to get the CIDR prefix, wildcard mask and usable hosts.

By TheCalcTools Updated Formula checked and tested

Your numbers

Your results

CIDR prefix
CIDR prefix
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Wildcard mask
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Addresses per subnet
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Usable hosts
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Equivalent /24 networks
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Subnet Mask to CIDR Prefix Calculator by TheCalcTools

How the Subnet Mask to CIDR Prefix Calculator works

Enter the four octets of the subnet mask.

A valid mask is a run of 1 bits followed by a run of 0 bits. The number of 1 bits is the prefix length.

The wildcard mask is the inverse of the mask, used in Cisco and Juniper ACLs.

The formula

Prefix = 32 − log₂(256⁴ − mask)Addresses = 2^(32 − prefix)

Worked example

Using the default values shown in the calculator:

Octet 1255
Octet 2255
Octet 3255
Octet 4192
  1. 255.255.255.192 has 26 one-bits, so the prefix is /26.
  2. The block holds 64 addresses, 62 usable. The wildcard is 0.0.0.63.

Result:

CIDR prefix/26
Wildcard mask0.0.0.63
Addresses per subnet64
Usable hosts62
Equivalent /24 networks0.2500

Frequently asked questions

What is 255.255.255.0 in CIDR?

/24, which gives 256 addresses and 254 usable hosts.

Why is 255.0.255.0 rejected?

A valid mask cannot have a 1 bit after a 0 bit.

Sources and further reading

IPv4 Subnet CalculatorEnter an IPv4 address and prefix length to get the network, broadcast, mask and host… CIDR Prefix to Hosts CalculatorPick a prefix length and see the address count, usable hosts and mask IP Address to Binary, Hex and Decimal ConverterConvert any IPv4 address into binary, hex and decimal Hosts to Subnet Prefix (VLSM) CalculatorEnter hosts needed and growth to get the right subnet size Subnet Split Calculator (Equal Subnets)Divide a parent network into the number of equal subnets you need

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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.