Be aware that this site doesn't do what it claims: "See which web sites are sharing the same IP Address." The truth would be: "See which web sites that have previously been searched for on our site that shares the same IP"

Basically it will only show sub-domains within the domain you search for, unless previous searches for other domains happen to match the IP.

I tested this just to confirm: I searched for one of my domains (apr. 20 domains on the same IP) and only sub-domains showed up. I searched for another of my domains and the sub-domains+my previous search showed up.