#!/usr/bin/perl

use strict;
use warnings;
use Net::DNS;

my $res = Net::DNS::Resolver -> new;
my $net = "81.86";

foreach my $c (0x00 .. 0xFF) {
    foreach my $d (0x00 .. 0xFF) {
        my $ip = "$net.$c.$d";
        print $ip unless $res -> query ($ip => 'PTR')
    }
}

__END__