Under construction
Often while change or updating the sites it is important to keep the visitors away. 
The Drupal-way is of course a great and build in way, but it has some down sides.
- visitors who know /user/login will still see much of the site. 
 
- the headers and footers are still visible
 
- and as a developer you have difficulties testing the site in real view situation
 
 
An alternative is to place this
 
$ip = $_SERVER['REMOTE_ADDR';]; 
if($ip != '94.212.157.4') { 
  header("Location: /underconstruction.html");  }
 
 
 
in the index.php. 
And set the ip address to your address. You can even add the underconstruction.html to inform visitors.
tip
Use a proxy to check if it is working properly