To allow only certain IP addresses access to a development website and direct all other traffic to a maintenance or other page you can achieve this with the .htaccess file in your webroot directory.

The following code can be used in a root level .htaccess file of the web site.

Create a ‘maintenance.html‘ or other named  file and save in the web root level or pass in an absolute URL.

---

Copy and paste the IP address line in the code for multiple IP addresses

<IfModule mod_rewrite.c>
 RewriteEngine on
 RewriteCond %{REMOTE_ADDR} !^123.456.789.000
 RewriteCond %{REQUEST_URI} !/maintenance.html$ [NC]
 RewriteCond %{REQUEST_URI} !\.(jpe?g?|png|gif) [NC]
 RewriteRule .* /maintenance.html [R=302,L]
</IfModule>

To allow multiple IP addresses just copy and paste the line and change the IP address.

Change the destination target to suit either a relative or absolute URL will work, maybe a redirect back to the real production site.