A 'PTR' record or pointer record maps an ipv4 address to the canonical name to the host. Setting up a PTR record for a hostname in the in-addr.arpa domain that corresponds to an ip address implements reverse lookup for that address. For eg www.name.net has the ip address 122.0.3.16 but a PTR record maps 16.3.0.122.in-inaddr.arpa
eg:
16.3.0.1222.in-addr.arpa. IN PTR name.net.
