Reverse DNS Check tool queries the given IP to resolve to a hostname. The hostname is anything like a normal domain or sub-domain, i.e. google-public-dns-a.google.com. This hostname is Google's hostname against IP 8.8.8.8 which is Google's public DNS ip. Whereas if you enter your ip, it might point to the hostname of your ISP, or if you query your server's IP, it might show you your domain name on which it resolves. This tool is named "Reverse DNS" just because it is a reverse process of resolving the hostname to IP just like a regular DNS check.