CIDR / Subnet Calculator
IPv4 and IPv6 CIDR calculator — network, broadcast, subnet mask, host range, RFC 1918 / ULA detection, and subnet splitting
Presets:
Notes: All math is 100% client-side. IPv4 uses BigInt-safe 32-bit arithmetic — RFC 3021 point-to-point links (/31) correctly report 2 usable hosts. IPv6 addresses are canonicalized per RFC 5952 (lowercase, longest-zero-run compression). Host counts for /64+ IPv6 prefixes display in both full decimal and scientific notation.
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