#!/usr/bin/perl use Socket; my $ADDR_PAT = /^\d+\.\d+\.\d+\.\d+$/; while (<>) { chomp; die "$_: Not a valid address" unless /$ADDR_PAT/o; my $name = gethostbyaddr(inet_aton($_),AF_INET); $name ||= '?'; print "$_ => $name\n"; }