§ cidr

cidr.

Subnet math for IPv4 and IPv6, with the address rendered in binary so the boundary between network and host bits is something you can actually see — not a number you trust the calculator about.

accepts

any IPv4 (10.0.0.0/8) or IPv6 (2001:db8::/48, ::1/128) in standard CIDR notation. embedded IPv4 (::ffff:1.2.3.4) is parsed into the equivalent v6.

classifies

RFC 1918 private space, CGNAT, link-local, loopback, multicast, IETF docs ranges (TEST-NET-1/2/3), reserved space, IPv6 ULA, Teredo, 6to4. labels follow RFC nomenclature.

binary

warm bits are part of the network prefix; dim bits are the host portion. for IPv6, the 128 bits are grouped by 16 to match the canonical hextet boundary.

share

the input is reflected in the URL on submit. result page is a permalink to the same calculation.