CIDR usable hosts calculator

It accepts ordinary subnet notation such as 192.168.1.0/24 and also normalizes an address entered from inside a block. Special-purpose /31 point-to-point networks and /32 host routes are counted correctly instead of blindly losing two addresses. The calculation is deterministic, local, and suitable for quick planning as well as automated validation.

Read total addresses and usable hosts correctly

An IPv4 prefix tells you how many of the 32 address bits identify the network. The remaining bits identify addresses inside that network, so the total is two raised to the number of remaining bits. For example, a /24 leaves eight bits and therefore contains 256 addresses. In conventional subnets from /0 through /30, the first address identifies the network and the last is the broadcast address. Those two addresses are not normally assignable to hosts, which leaves 254 usable hosts in a /24. This calculator reports both numbers because they answer different planning questions: total addresses describes the allocated block, while usable hosts describes ordinary interface capacity. It also returns the normalized network and broadcast boundaries, making it easy to verify whether an address belongs where you expect. The reserved count makes the subtraction explicit. Counts are exact integers, including the 4,294,967,296 addresses represented by the complete IPv4 /0 space, so the result is useful for inventories, documentation, and validation rules without relying on a table of memorized subnet sizes.

Understand the special rules for /31 and /32

The familiar rule of subtracting two addresses has two important exceptions. A /31 contains exactly two addresses. On a point-to-point link, there is no need for separate network and broadcast destinations, so both addresses can identify the two endpoints under the widely deployed /31 convention. The calculator therefore reports two total and two usable addresses, with zero reserved. A /32 contains one address and represents a single host route rather than a conventional multi-host subnet. That sole address is usable for the represented host, so the result is one total, one usable, and zero reserved. Treating either prefix with a mechanical subtraction would produce zero or even a negative answer, which is misleading in routing plans and automation. The output still includes network and broadcast fields for a consistent response shape: for /32 both are the same address, while for /31 they are the two endpoints. Always consider equipment support and local policy before deploying /31 links, but use these counts when modeling modern IPv4 semantics.

Use normalized boundaries in network planning

You may enter any IPv4 address with a prefix, not only an address already written at the network boundary. If you submit 192.168.1.37/24, the calculator normalizes it to 192.168.1.0/24 and reports 192.168.1.255 as the broadcast address. This behavior is valuable when reviewing firewall objects, host inventories, cloud network plans, or configuration generated by another system: it exposes the actual block instead of echoing a potentially misleading host address. Validate user input before provisioning, compare the returned normalized CIDR with the intended allocation, and use usable_hosts for capacity checks. Remember that a mathematically usable address may still be withheld by a cloud provider, appliance, or organizational policy; provider-specific gateway and service reservations are outside this generic IPv4 calculation. The endpoint performs no DNS lookups and does not inspect a live network. It only parses dotted-decimal IPv4 CIDR notation and applies deterministic bit-boundary arithmetic. That makes repeated results stable and safe to include in build pipelines, infrastructure review tools, address-management imports, and educational subnet exercises.