The top-level domain containing in-addr.arpa for reverse DNS lookups is .arpa.
Reverse DNS performs the opposite of ordinary DNS: instead of finding an IP address from a hostname, it finds a hostname associated with an IP address. IPv4 reverse lookups use in-addr.arpa, while IPv6 uses the related ip6.arpa domain.
The address is written in reverse order beneath in-addr.arpa. For example, a lookup for 8.8.4.4 queries 4.4.8.8.in-addr.arpa, whose PTR record can identify a hostname. This reversed structure follows DNS delegation boundaries and lets different network operators manage their address ranges.
.arpa began as a temporary domain associated with ARPANET, the network that preceded today’s Internet. Its name originally referred to the Advanced Research Projects Agency and was later reinterpreted as “Address and Routing Parameter Area.” It is an infrastructure domain, not a normal public-registration domain; .net and .int serve different purposes.