CIDR to Subnet Mask Converter

Reading CIDR notation

CIDR notation writes the mask as a single number: the count of leading one-bits. /24 means 24 one-bits then 8 zero-bits, which as a dotted address is 255.255.255.0. The two notations carry exactly the same information.

The one-bits must be contiguous. 255.255.255.192 is a valid mask because its bits run 1…10…0; 255.255.0.255 is not, and no prefix length corresponds to it. Non-contiguous masks were possible in theory long ago and are rejected by every modern stack.

Converting in your head

Split the prefix by octets. Every full group of 8 bits is a 255, and the remainder sets the next octet from the fixed sequence 128, 192, 224, 240, 248, 252, 254, 255 — one entry per leftover bit.

So /26 is 24 bits (255.255.255) plus 2 more, and the second entry in that sequence is 192: 255.255.255.192. The same sequence read backwards gives the wildcard mask, which is what remains of 255 in each octet.

/20 → 16 bits = 255.255 · 4 bits left → 240 · mask = 255.255.240.0

Where each form is expected

CIDR notation is what routing tables, firewall rules, cloud security groups and Kubernetes manifests use. Dotted masks are what interface configuration on Windows, older router CLIs and many DHCP scopes still ask for.

Wildcard masks are a third form, used by Cisco access lists and OSPF network statements. They are the bitwise inverse of the subnet mask, which is why /24 appears there as 0.0.0.255.

Most-used prefixes

Frequently asked questions

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