Check whether or not a CIDR subnet contains an IP address

I'm looking for quick/simple method for matching a given IP4 dotted quad IP to a CIDR notation mask.

I have a bunch of IPs I need to see if they match a range of IPs.

example:

$ips = array('10.2.1.100', '10.2.1.101', '10.5.1.100', '1.2.3.4');

foreach ($ips as $IP) {
    if (cidr_match($IP, '10.2.0.0/16') == true) {
        print "you're in the 10.2 subnet\n"; 
    }
}

What would cidr_match() look like?

It doesn't really have to be simple, but fast would be good. Anything that uses only built-in/common functions is a bonus (as I'm likely to get one person to show me something in pear that does this, but I can't depend on pear or that package being installed where my code is deployed).

Solution

If only using IPv4:

use ip2long() to convert the IPs and the subnet range into long integers
convert the /xx into a subnet mask
do a bitwise 'and' (i.e. ip & mask)' and check that that 'result = subnet'

something like this should work:

function cidr_match($ip, $range)
{
    list ($subnet, $bits) = explode('/', $range);
    if ($bits === null) {
        $bits = 32;
    }
    $ip = ip2long($ip);
    $subnet = ip2long($subnet);
    $mask = -1 << (32 - $bits);
    $subnet &= $mask; # nb: in case the supplied subnet wasn't correctly aligned
    return ($ip & $mask) == $subnet;
}