jdresolve resolves IP addresses to hostnames. Any file format is
supported, including those where the line does not begin with the IP
address. One of the strongest features of the program is the support
for recursion, which can drastically reduce the number of unresolved
hosts by faking a hostname based on the network that the IP belongs
to. DNS queries are sent in parallel, which means that you can
decrease run time by increasing the number of simultaneous sockets
used (given a fast enough machine and available bandwidth). By using
the database support, performance can be increased even further, by
using cached data from previous runs.

Included is a tiny shell script called rhost to interface with
jdresolve when resolving a single IP address. Think of it as a smart
replacement for the 'host' utility that comes with bind-utils.

WWW: http://www.jdrowell.com/archives/projects/jdresolve/index.html