The process, however, can only be completed after the ISP provider has pointed the zone to the IP’s DNS server system. The PTR record then becomes available for reverse DNS entries, which can map IPv4 or IPv6 addresses to the host’s canonical name. Reverse DNS works by looking up query DNS servers for a pointer record . A PTR record maps an IPv4 or IPv6 address to the canonical name for the host. If there is no PTR record on the server, it cannot resolve a reverse lookup. A web server can be configured to serve 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 a great way to expand the attack surface during web server reconnaissance. If, for example, your primary target website appears secure, you may be able to gain access to the underlying operating system by attacking a less secure site on the same server. Potentially bypassing the target site’s security controls. Overblocking is a problem known to affect filtering in China, Vietnam, Saudi Arabia, and other countries that employ government-mandated country-wide web filtering policies. Additionally, research has indicated that IP address filtering is used by many commercial web filters installed in libraries and schools in the United States. Sometimes Internet services providers are legally required to implement IP address level filtering. For example, under 2002 law, the Attorney General of Pennsylvania ordered Internet service providers in Pennsylvania to disable access to sites found to offer child pornography. Hosts with poor reputation can affect email delivery, blacklisting of your site, and search engine ranking. Use the reverse IP address lookup service to identify other sites on your host. Next, use investigative tools to identify if these other hosts are of poor quality, perhaps even spam or phishing sites. It is also common in many organizations and can be an excellent way to expand the attack surface during reconnaissance of 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.