How does reverse IP Lookup work?

Reverse IP lookup tries to determine which domains are hosted on the same ip address. 

You may ask why this happens?

This is because a lot of times, domains are being hosted with some hosting company which puts a lot of them on the same server.

There is another reason this may happen, namely many domains having same IP. 

E.g. domains may be behind security/CDN providers like Cloudflare which then may share the same IP address again.

How do we then determine the domains on same IP?

This can be done by doing exhaustive domain to IP mapping and then doing some SQL analysis of this data, which produces the domains on same IP.