CIDR / Subnet Calculator
Break down any IPv4 CIDR block into network and broadcast addresses, usable host range, host count, and netmask, and see which IANA special-purpose range it falls in. The other modes carve variable-length subnets (VLSM) out of a parent block, summarise prefixes into a supernet, and find overlaps and gaps. Runs entirely in your browser.
NetworkingCIDR Calculator
Analyze a single IPv4 CIDR block: network, broadcast, mask, host range, and the special-purpose ranges it falls in or overlaps.
Runs locally. Nothing is sent to any server.
What this page needs
This tool computes entirely on your device, so nothing you type is sent anywhere. That local computation needs JavaScript; CSS is used for layout and theming.
This tool is built to also work over an HTTP API. Processing is currently off, so the API is documented but not served; when it is switched on, you will be able to call it without a browser.
All results are computed locally from the blocks you enter, using standard IPv4 addressing arithmetic: one block's network, mask, host range and counts (a /31 counts two usable hosts, RFC 3021); variable-length subnets carved from a parent, largest block first, with the free space left as blocks; the minimal aggregate and single supernet of a list; and the overlaps and gaps among prefixes. The special-purpose classification compares a block with an embedded copy of the IANA IPv4 Special-Purpose Address Space registry, read on 2026-10-19, plus the multicast range 224.0.0.0/4; nothing is looked up remotely and nothing is sent anywhere. Behaviour is pinned by 26 golden vectors.
- RFC 4632 (CIDR)Classless Inter-Domain Routing: prefix notation, block sizes and aggregation
- RFC 1122, section 3.2.1.3The network and broadcast addresses a /0 to /30 block reserves
- RFC 302131-bit prefixes on point-to-point links: both addresses are hosts
- IANA IPv4 Special-Purpose Address SpaceEvery row of the special-purpose classification, with its flags (registry updated 2025-10-09, read 2026-10-19)
- RFC 6890Special-Purpose IP Address Registries: the registry's structure and flag columns (updated by RFC 8190)
- RFC 1918Private address ranges
- RFC 791 (IPv4)The Internet Protocol