How reverse DNS works
DNS isn't only used to turn names into addresses — there's a parallel zone tree under in-addr.arpa (IPv4) and ip6.arpa (IPv6) that maps addresses back to names. To look up 8.8.8.8, the resolver issues a PTR query for 8.8.8.8.in-addr.arpa. IPv6 uses nibble-reversed form, e.g. 2606:4700::1111 becomes 1.1.1.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.7.4.6.0.6.2.ip6.arpa.
JSON API
POST /api/reverse-dns with body { "ips": ["8.8.8.8", "1.1.1.1"] }. CORS-enabled, no auth. Same 100-IP cap. See the tools index for sibling utilities.