nslookup

As described by our associate Wedge, nslookup is a tool to query name servers. It is found on both UNIX and Windows NT. Though nslookup can perform a variety of tasks, in my experience it is used primarily to detect to problems with network installations by trying to lookup the IP address of machines on the network. The most commonly used feature of nslookup, in indeed to retrieve the IP associated with a hostname.

Also very useful for getting hostname by IP. For instance, if you have someone's IP and want to know their ISP, and sometimes where in the world they are, do an nslookup.