Reverse DNS for IPv4 is specified in RFC1035 Section 3.5. The special
in-addr.arpa domain was created for this. For IPv6 the special domain is
ip6.arpa as specified in RFC3596. The record type that gets requested for reverse lookup is the
For IPv4 the IP address is reversed on the "dot" boundaries after every byte:
192.168.0.1 gets transformed into
For IPv6 the address gets reversed after every 4-bit "nibble" (every number/letter) with a dot inserted:
2001:db8::dead:beef gets transformed into
For DNS servers there is no extra work involved, every DNS server software understands/supports PTR records. The reverse lookup on the client is done by the application(s) you use. There are standard system calls for reverse lookup on every OS.