CIDR Chart: Prefix, Mask and Host Counts

Every IPv4 prefix length with its mask, wildcard, and host count. Source of the arithmetic is the addressing model itself, so these figures do not change; the routing and allocation notes below reflect current practice as of 2026.

Full chart, /0 to /32

Need a specific block rather than a row? The subnet calculator returns network, broadcast, and usable range for any input and can split a block into equal subnets.

How to read the numbers

Usable hosts is total minus two for every prefix from /0 to /30, because the first address identifies the network and the last is the broadcast address. Neither is assignable to an interface.

The two exceptions are /31 and /32. RFC 3021 defines /31 for point-to-point links where no network or broadcast address is reserved, so both addresses are usable — this is why /31 rather than /30 is the current default for router-to-router links. A /32 is a single host route, used for loopbacks and anycast service addresses.

Each additional bit halves the block. A /23 is two /24s, a /22 is four, a /21 is eight. The pattern is the fastest way to answer allocation questions without a calculator.

Prefix lengths that matter operationally

/24 is the practical floor for global routing. Most networks filter IPv4 announcements longer than /24, so a /24 is the smallest block that propagates reliably on the public internet. Anything longer is usable internally but will not be seen by most of the default-free zone.

/24 is also the smallest transferable unit at most registries. RIR policy and market practice both settle at /24, which is why block sizes in transfers are quoted as /24, /22, /21, /20 and so on.

/29 and /28 are the common customer-facing assignments in hosting and business access, giving six and fourteen usable addresses respectively — enough for a handful of services plus the gateway.

/22 and /21 are the usual first purchases for a growing operator, because they aggregate into a single announcement while leaving room to split per site or per service later. How to divide them without painting yourself into a corner is covered in address plan design.

Related

• Subnet calculator — compute and split a specific block
• Subnet mask cheat sheet — the same data condensed for printing
• Reserved and bogon ranges — which of this space never routes
• Glossary — terms used above

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