what is reverse DNS
There are two types of dns: forward dns and reverse dns. Forward dns is the one we all use on a very regular basis. It allows us to use the internet in a human friendly way, otherwise we would have to remember a set of numbers known as an IP address. Forward dns takes a domain request such as "google.com", goes out to the world and returns back with an ip address your computer can use.
Reverse dns does the opposite, you query with an ip address for it's assigned domain name. It is commonly used to provide extra blocking measures for spam emails.