nslookup is the name of a program that lets an Internet server administrator or any computer user enter a host name (for example, "whatis.com") and find out the corresponding IP address. It will also do reverse name lookup and find the host name for an IP address you specify.
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For example, if you entered "whatis.com" (which is one of the TechTarget sites), you would receive as a response our IP address, which happens to be :
Or if you entered "220.127.116.11", it would return "sites.techtarget.com".
nslookup sends a domain name query packet to a designated (or defaulted) domain name system (DNS) server. Depending on the system you are using, the default may be the local DNS name server at your service provider, some intermediate name server, or the root server system for the entire domain name system hierarchy.
Using the Linux and possibly other versions of nslookup, you can locate other information associated with the host name or IP address, such as associated mail services. nslookup is included with some UNIX-based operating systems and in later Windows systems. In Windows XP, the command can be entered on the "Command prompt" screen. A more limited alternative to nslookup for looking up an IP address is the ping command.