mask_ips.php

<?php
function less_ip($ip) {
        $ipv6 = (strstr($ip, ':') !== false);

        $in_addr = inet_pton($ip);

        if ($ipv6) {
                // Not sure how many to mask for IPv6, opinions?
                $mask = inet_pton('ffff:ffff:ffff:ffff:ffff:0:0:0');
        } else {
                $mask = inet_pton('255.255.0.0');
        }

        $final = inet_ntop($in_addr & $mask);
        return str_replace(array(':0', '.0'), array(':x', '.x'), $final);
}

function less_hostmask($hostmask) {
        $parts = explode('.', $hostmask);

        if (sizeof($parts) < 3)
                return $hostmask;

        $parts[0] = 'x';
        $parts[1] = 'x';

        return implode('.', $parts);
}