Convert a Classless Inter-Domain Routing prefix to a plain old network mask.

cidr_prefix_to_netmask.php

<?php

/*
 * Convert a Classless Inter-Domain Routing prefix to a plain old network mask.
 *
 * E.g.: 24 => 255.255.255.0
 */

function cidr_prefix_to_netmask($prefix)
{
    for ($i = 0, $netmask = 0; $i <= (int)$prefix; $i++, $netmask += 2 << (32 - $i));

    return long2ip($netmask);
}