title: How to validate an invalid CIDR block?
body: <p>I have the following CIDR reserved for my VPC -> <code>10.0.0.0/22</code></p>
  
  <p>What this tells me is that I have 10 bits leftover for my host ips or <code>1,024</code> addresses. I was attempting to compute the <em>range</em> of valid addresses, as Amazon asks for a valid IPv4 CIDR block when creating a new private subnet. </p>
  
  <p>I figured 6/8 bits are taken from the 3rd octet and the last octet is all 0's, which leads me with an IP range of <code>10.0.0.0 -&gt; 10.0.3.255</code>. </p>
  
  <p>As I come to the screen to actually pick my IPv4 CIDR block, I'm getting an "Invalid error", which is just validating that I don't understand how the math is actually working work. I typed in <code>10.0.2.1/28</code> which yields a <strong>Must be valid Ipv4 CIDR</strong> error. </p>
  
  <h3>My thought process:</h3>
  
  <p>This looked like it was in the range I had calculated and that I wanted my private subnet to reserve 16 IP addresses.</p>
  
  <p>What am I doing wrong?</p>