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CIDR

published · September 10, 2026

CIDR notation writes a range of IP addresses as one address and a prefix length, so 203.0.113.0/24 means the 256 addresses sharing those first 24 bits.

CIDR notation writes a range of IP addresses as one address and a prefix length: 203.0.113.0/24 means the 256 addresses that share those first 24 bits. Classless Inter-Domain Routing replaced the old fixed classes in 1993, and every allocation, route and blocklist has been written this way since.

Where it appears in Spaw

network on an IP answer is the CIDR block the address was found in — the unit the registries allocate and the feeds list, so it is the right thing to remember if you are caching or aggregating.

CIDR is also the input format in two places: a secret API key can carry an allowlist of source addresses or ranges, and every threat and datacenter feed read here is compiled from published CIDR lists.

Why a range and not an address

Almost nothing about an IP address is a fact about the address alone. Allocation, hosting, blocklisting and geolocation are all properties of a block, and an answer that pretends otherwise is claiming a precision the data does not have. location_confidence says which of country, region or city the underlying data actually supports for a given block.

Related

markdown version: /glossary/cidr.md

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More glossary

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  • Check digit

    A check digit is a digit computed from the rest of an identifier so that a mistyped one usually fails arithmetic before anybody looks it up.

  • Company number

    A company number is the identifier a national registrar assigns to an incorporated company, unique within that register and never within the world.

  • Credit

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  • Delivery point

    A delivery point is the smallest unit a postal service delivers to: one letterbox, one door, one box at a counter.