Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses.
Major search engines use a Reverse DNS lookup to verify sites before they're indexed by the searched engines. Sites with domain names that don't match their IP addresses are either rejected outright, or penalized. Those sites are usually hosted with a shared IP address in which multiple websites utilize the same IP address. Our hosting accounts include a unique IP address with Reverse DNS capability for every domain name.