This is a simple, very easy to use Maintenance Mode plugin that allows you to put your site into “maintenance mode” in order to perform updates or other changes. The plugin allows you to specify a set of authorized IP addresses for people that will be allowed to see the site normally, and also to set a redirect location for unauthorized visitors.

Demonstration

From the plugin settings page, you can quickly enable maintenance mode, set the authorized IP addresses, and choose where to redirect visitors who do not have access, either to a WordPress page or external URL.