ip6.arpa
The DNS zone used for IPv6 reverse lookups, where each nibble of the address becomes a label.
`ip6.arpa` is the IPv6 equivalent of `in-addr.arpa`. Each of the 32 hexadecimal nibbles of an IPv6 address is reversed and turned into a label, so `2001:4860:4860::8888` becomes `8.8.8.8.0.0.0.0.0.0.0.0.0.0.0.0.0.6.8.4.0.6.8.4.1.0.0.2.ip6.arpa`, holding a PTR record. IPv6 reverse zones are delegated in /4 increments by RIRs and large allocation holders. Many IPv6 deployments skip reverse DNS entirely or auto-generate placeholder records, which can cause mail rejection if FCrDNS is enforced.