Problem:
You need to hide or block the access to your Magento web site, but that block need to be conditional for only some IP address.
Solution:
Since the version 1.4 is possible to activate a Maintenance Mode just creating a file called maintenance.flag in your root directory but that is not really useful at all.
You need a workaround, to allow the access to some IP adress:
1. Open index.php in the root .
2. Look for that line:
if (file_exists($maintenanceFile)) {
3. Replace it with that one:
if (file_exists($maintenanceFile) && !in_array($ip, $allowed)) {
4. Paste next code just above the line before:
$ip = $_SERVER['REMOTE_ADDR'];
$allowed = array('0.0.0.0','9.9.9.9');
5. In the $allowed array you have to included the IP allowed to access to your website.