The DNS is used to determine what IP address is associated with a given host name, so to reverse resolve a known IP address is to lookup what the associated host name for it. A reverse lookup is often referred to simply as reverse resolving, or more specifically reverse DNS lookups. Use of reverse DNS. Anti-spam Network troubleshooting Avoid spammers and phishers using a forward confirmed reverse DNS etc Check reverse DNS for given IP address under Linux or Windows XP/Server 2003. You can use standard UNIX / Linux utilities such as nslookup, dig or hosts to find out reverse DNS of a given IP address. eg; under Linux/UNIX/ $ host 66.65.123.34 or dig -x IP (66.65.123.34) and under Linux/UNIX/Windows nslookup 66.65.123.34