To add a new host to your zone files: Edit the appropriate zone file for the domain the host is in. Add an entry for each address of the host. Optionally add CNAME, HINFO, WKS, and MX records. Add the reverse IN-ADDR entry for each host address in the appropriate zone files for each network the host in on.
Told you! :-) The reverse DNS lookup looks for a PTR entry in the DNS database in the in-addr.arpa zone for IPv4 addresses.