Subnet usable host calculator

It reports the normalized prefix, the equivalent mask, the total number of addresses in the subnet, and the usable host count after excluding the network and broadcast addresses. Use it when planning address space, checking an exam answer, documenting a network, or validating an infrastructure configuration. The calculation is deterministic and requires no network lookup, so the same input always produces the same result.

Enter either a CIDR prefix or subnet mask

Provide a prefix as 24 or /24, or enter a dotted-decimal mask such as 255.255.255.0. The calculator accepts every IPv4 prefix from 0 through 32, including the boundary values. When you supply a dotted mask, each octet must be between 0 and 255 and its binary form must be contiguous: all one bits must come before all zero bits. A value such as 255.0.255.0 is rejected because it cannot represent a conventional CIDR subnet mask. The result normalizes both accepted forms into a prefix length and a dotted mask, which makes it easy to compare configuration files that use different notation. Whitespace around the value is ignored, but extra text, IPv4 addresses with an attached prefix, negative numbers, decimals, and prefixes greater than 32 are intentionally rejected. This strict input behavior helps prevent a plausible-looking capacity result from being produced for an ambiguous or mistyped network setting. Use the subnet, prefix, or mask field name when calling the API; subnet is the primary documented field.

Understand total addresses and usable hosts

An IPv4 address has 32 bits. A prefix fixes the specified number of leading network bits and leaves the remaining bits available for addresses inside the subnet. The total is therefore two raised to the power of 32 minus the prefix length. For /24, eight bits remain, producing 256 total addresses. This capability follows the requested traditional host-count convention and subtracts two addresses: one for the network identifier and one for the broadcast address. A /24 consequently reports 254 usable host addresses. Very small subnets need special attention. Because the calculation always excludes network and broadcast, /31 and /32 return zero usable host addresses here. Some point-to-point deployments use the two addresses of a /31 under a special standards-based convention, and a host route identifies one address with /32, but those operational interpretations are outside this calculator’s stated rule. The explicit total remains useful: /31 has two total addresses and /32 has one. The largest accepted network, /0, has 4,294,967,296 total addresses and 4,294,967,294 after the two exclusions.

Apply the result to network planning

Use the usable count as an initial capacity check, then account for addresses reserved by your actual environment. Gateways, load balancers, DNS services, appliances, virtual IPs, cloud-provider reservations, and growth margins can all reduce the number available to endpoints beyond the network and broadcast exclusions performed here. For example, a /26 provides 64 total addresses and 62 under the traditional usable-host formula, but it may support fewer than 62 application instances after platform reservations. The normalized mask is also useful when translating a CIDR-based design into equipment or documentation that expects dotted-decimal notation. In automation, compare the returned usable_host_addresses value with the number of required devices and fail deployment early when capacity is insufficient. Store the prefix_length and subnet_mask alongside that decision so reviewers can see exactly which interpretation was used. This tool counts addresses only: it does not calculate a network address or broadcast address because no host IP is supplied, and it does not inspect routes, VLANs, DHCP pools, IPv6 prefixes, or live equipment. API execution costs $0.002 per request, while the deterministic calculation remains suitable for quick interactive checks.