How can you view a live site while developing, but hide it from others?

From Joomla! Documentation

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Introduction

The method described below should be used for relatively minor modifications, such as adjusting menus or quickly reorganizing content sections. More complex tasks, such as installing new components or adjusting complex configuration settings should be performed and tested on a development server first. Not only does this keep your public site up and running, but it also lets you test at your leisure, thus reducing errors. One way to do it is to create a sub-domain (i. e., dev.yourdomain.com) and install Joomla! there just as it is installed on your public site.

See also Taking the website temporarily offline.

Directions

  1. Log in to the Backend, and choose: Site    Global Configuration.
  2. The first option you'll see is is to set the site offline. Choose "Yes" and press the Save button. This will prevent display of all site pages and replace them with the following message:
    This site is down for maintenance. Please check back again soon.
    
    • Administrators can still log into the site from the maintenance page to preview changes
  3. While you are logged into the Backend administrator system, you can still view the Frontend, by
    • clicking the site name in the top right corner
    This will display the site as it would appear to users.