IPv6 Subnet Calculator

Because SLAAC — the stateless auto-configuration that lets devices generate their own addresses — is defined to work only with 64-bit interface identifiers. A /64 gives every LAN 18.4 quintillion addresses, which feels absurd, but the design goal was never address efficiency: it was making every subnet the same size so renumbering, auto-configuration and privacy extensions just work.

IPv6 replaced broadcast with multicast. Instead of shouting at every host on the subnet, protocols address well-known multicast groups: ff02::1 reaches all nodes on the link, ff02::2 all routers. That's also why IPv6 subnet math has no "minus 2" — the all-zeros address is the Subnet-Router anycast, but in practice all 2^(128−prefix) addresses in a /64 are usable for hosts.

Each bit of prefix difference doubles the count: a /48 contains 65,536 /64s (16 bits of subnetting space) and a /56 contains 256. That's why ISPs commonly delegate a /56 to home users (256 LANs) and a /48 to businesses. If your provider hands you a single /64, you can't subnet it without breaking SLAAC — push back and ask for a /56.

Unique Local Addresses (fc00::/7, in practice fd00::/8 with a random 40-bit global ID) are IPv6's equivalent of 10.0.0.0/8 — routable inside your organisation, never on the public internet. The random global ID means two merged company networks almost certainly won't collide, which was a chronic problem with everyone picking 10.0.0.0/8. Unlike IPv4, hosts typically hold a ULA and a global address simultaneously.

RFC 5952 defines the canonical form: lowercase hex, leading zeros dropped in each group, and the single longest run of two or more zero groups replaced with :: (leftmost wins on a tie). So 2001:0DB8:0000:0000:0000:0000:0000:0001 canonically becomes 2001:db8::1. Only one :: is allowed per address — two would be ambiguous.

Reverse DNS for IPv6 reverses the address one hex nibble at a time and appends .ip6.arpa — so 2001:db8::1 becomes a 32-label name ending in 8.b.d.0.1.0.0.2.ip6.arpa. It's tedious to write by hand (this calculator generates it for you), and mail servers in particular still expect a working PTR record.

A /127 is the recommended size for point-to-point links between routers (RFC 6164) — exactly two addresses, and it prevents the ping-pong and neighbor-cache exhaustion attacks a /64 link would allow. A /128 is a single host route: loopbacks on routers, VIPs, and firewall rules pinning one exact address.

Yes — the IPv4 version lives at its own page with binary breakdowns, wildcard masks and an IPv4-specific splitter. IPv4 math has usable-host and broadcast rules that don't exist in IPv6, which is why the two calculators are separate tools.