Joomla Block Users From Specific Domains : IP to Domains

Locate all domain names hosted on a single IP , Joomla block users from specific domains

Type an IP address or domain name into the form and click "OK" to locate a full list of domain names hosted on a single IP address.

Shared Hosting accounts are in most cases placed under the same IP address. See who is using that shared server before signing up or see who else is on your shared host.
Parking Sites can be identified by the large number domains parked on their IP address. See who else is parked on that server.
Scam websites in most cases come together. If you find one do a IP to Domain look up to find the other scam sites hosted on the same server as the one you found.