title: AWS: EC2 in public subnet can't ping EC2 in private subnet
body: <p>Very strange:</p>
  
  <ul>
  <li>The public and private subnets are in the same VPC.</li>
  <li>EC2 in private subnet uses a security group that opens up 0-65535 to 0.0.0.0/0. So does its network ACL.</li>
  <li>I can RDP from the <strong>public</strong> subnet EC2 into the <strong>private</strong> subnet EC2.</li>
  <li>I can ping between <strong>private</strong> subnet EC2 instances.</li>
  <li>But I just can't <strong>ping</strong> from the <strong>public</strong> subnet EC2 to the <strong>private</strong> subnet EC2.</li>
  </ul>