So, do you know the IPs? If you don't, I'm not sure there is a way to do it.
<?php
$ipaddress = $_SERVER['REMOTE_ADDR'];
if ($ipaddress == <first IP> || <second IP> || <third IP> || <fourth IP>)
{
header("Location: /secret_area/index.php"); // change to where you want to send them if IP is matched
}
else
{
header("Location: /index.htm"); // change to homepage
}
?>