title: Why does Azure Application Gateway require an empty subnet
body: <p>When I try to execute New-AzureRmApplicationGatewayIPConfiguration to create an application gateway, I get an exception:</p>
  
  <pre><code>Subnet xxx cannot be used for application gateway yyy since subnet is not empty.
  </code></pre>
  
  <p>I encountered this error when I tried to add the application gateway to the same subnet as the backend servers.</p>
  
  <p>Why is this not an option? Does each gateway require a separate subnet? What is the recommended configuration?</p>
  
  <p>Related questions:</p>
  
  <ul>
  <li>The documentation says backend servers can be added when they belong to the virtual network subnet. How can a back-end server belong to the virtual network subnet of the application gateway if the application gateway must be in a separate subnet?</li>
  <li>How can the application gateway be configured without requiring a public IP address on the backend servers?</li>
  </ul>