#!/usr/bin/perl -w

use Socket;
$IP = "129.21.30.99";

# inet_aton returns an "opaque string" C struct 
#  that is passed to the system call.
$IPaddr = inet_aton( $IP );

# AF_INET means that we're interested in Internet objects
#  rather than Unix file system objects (AF_UNIX).
$hostName = gethostbyaddr( $IPaddr, AF_INET );

print "$IP: $hostName\n";