The DNS Lookup tool is designed to act just like a DNS client in the way that it gets information from other DNS servers. Because of this, all PTR lookups must be in the same notation that would be used in a standard query. The format is as follows:
<IP address in reverse>.in-addr.arpa
So as an example, looking up the PTR for 127.0.0.1 would be 1.0.0.127.in-addr.arpa. If you want to do the lookup using just an IP address, please use the Reverse DNS tool.