If you plan an maintenance shutdown for your website, you can use the mod-rewrite for the maintenance forward.
ReWriteEngine On
ReWriteCond %{REMOTE_ADDR} !^123\.123\.123\.([0-9]+)$
ReWriteRule ^(.*)$ http://your-site.com/maintenance.html [L]
This code will forward all users except from IP block 123.123.123.0/24 to an maintenance site.
If you have got 123.123.123.0/24 you can use the site as normal, and all other users will be forwarded.
