CIDR ↔ Subnet Mask Converter

Convert between CIDR notation like /24 and dotted-decimal subnet masks like 255.255.255.0.

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What this tool does

This tool converts CIDR prefixes into subnet masks and subnet masks into CIDR prefixes. It is useful when reading network diagrams, firewall rules, or cloud networking configuration.

When this tool is useful

Converting /24, /27, or /29 into dotted-decimal subnet masks
Checking what subnet mask a firewall or router entry corresponds to
Translating documentation between CIDR and subnet mask formats

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