(also rDNS)
Reverse DNS is a lookup method in the Domain Name System that determines the host of an IP address. It’s the opposite of forward DNS lookup that determines the IP address from a domain name.
If a website has active reverse DNS, you can access it by entering its IP address into the search bar instead of the domain name. So if you entered 173.194.217.103 into your browser’s search bar, you would reach Google.