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The technique known as Reverse IP Lookup is a way to identify hostnames that have DNS (A) records associated with an IP address.
A web server can be configured to server multiple virtual hosts from a single IP address. This is a common technique in shared hosting environments. It is also common in many organizations and can be an excellent way to expand the attack surface when going after a web server. If for example your primary target web site appears to be secure you may be able to gain access to the underlying operating system by attacking a less secure site on the same server. Bypassing the security controls of the target site.