The ptr mechanism is explained with:
First the <ip>'s name is looked up using this procedure: perform a DNS
reverse-mapping for <ip>, looking up the corresponding PTR record in
"in-addr.apra.". For each record returned, validate the host name by
looking up its IP address. If <ip> is among the returned IP addr :
It should be:
First the <ip>'s name is looked up using this procedure: perform a DNS
reverse-mapping for <ip>, looking up the corresponding PTR record in
"in-addr.apra." if the address is an IPv4 one and "ip6.arpa." if it is
an IPv6 address. For each record returned, validate the host name by
looking up its IP address (v4 is <ip> was v4 and v6 it is was v6). If
<ip> is among the returned IP addr :