/*ip address used here can only access wp-admin page all other ips will not be able to access it */

function block_all_user_except_allow() {
$allow = array("xx.xxx.xxx.xx","xx.xxx.xx.xx");

if (!in_array ($_SERVER['REMOTE_ADDR'], $allow)) 
{
    header("Location: https://yourwebsite.com");
    die();
}
}
add_action('login_init', 'block_all_user_except_allow');