title: AWS: how to create a private subnet to the default VPC
body: <p>I am following the <a href="http://docs.aws.amazon.com/gettingstarted/latest/wah-linux/web-app-hosting-intro.html" rel="nofollow">Hosting a Web App on Amazon Web Services</a> tutorial.</p>
  
  <p>It says that the EC2 instance should be launched into a public subnet. However, database Server should be launched into a private subnet. But when I try to create the private subnet, I have problem choosing CIDR. My default VPC is 172.31.0.0/16. If the CIDR is 172.31.x.0/16, it showed error that CIDR already existed.</p>