Subnet / CIDR Calculator
Parse IPv4 or IPv6 CIDR (or dotted netmask), expand to network address, broadcast, usable range, wildcard mask and binary. Split a parent CIDR into child subnets, or roll two adjacent subnets back into a supernet. All math runs client-side — nothing is sent to a server.
FAQ
What inputs does the calculator accept?
IPv4 in CIDR form (10.0.0.0/24) and IPv4 with a dotted netmask (10.0.0.0 255.255.255.0 or 10.0.0.0/255.255.255.0) — the netmask is converted to its prefix length internally. IPv6 is accepted in canonical compressed form with a prefix (2001:db8::/48, fe80::/64). Prefix lengths must fall in 0-32 for IPv4 and 0-128 for IPv6.
Why does my /31 or /32 not show a usable host range?
A /32 is a single host (no network or broadcast distinction). A /31 is a point-to-point link per RFC 3021 — both addresses are usable as endpoints but there is no broadcast. For /30 and shorter prefixes, the network address and the broadcast address are reserved, so the usable range is network+1 to broadcast-1.
How accurate is the IPv6 host count for a /48 or /64?
Exact. IPv6 host counts are computed with native JavaScript BigInt — a /64 reports 2^64 = 18,446,744,073,709,551,616 addresses with no floating-point rounding. Subnet split and roll-up operate on the same BigInt representation so even /127 to /128 math is byte-precise.
Can I split a /24 into smaller subnets?
Yes — the Split tab takes a parent CIDR and a target prefix length, then enumerates every child subnet of that size. Splitting 10.0.0.0/24 into /26 yields four subnets: 10.0.0.0/26, 10.0.0.64/26, 10.0.0.128/26 and 10.0.0.192/26. For IPv6 the same operation works up to a sane row cap (256 rows) so a /48 → /52 expansion does not lock the browser.
How does the supernet roll-up work?
Two subnets can be combined into a supernet only if they share the same prefix length and are adjacent on the next-shorter boundary — meaning the lower one's network address has its (prefix-1)-th bit clear and the upper one's network address is exactly lower + size. 10.0.0.0/25 and 10.0.0.128/25 roll up to 10.0.0.0/24. 10.0.0.0/25 and 10.0.0.192/26 do NOT roll up because the prefix lengths differ.
Does this tool send my IPs anywhere?
No. All parsing, splitting and roll-up math runs synchronously in the browser. There is no fetch, no analytics on the input fields, no third-party script with access to the values. You can verify by opening DevTools and watching the Network tab while you type.