title: What is the meaning of inbound source of default RDS security group?
body: <p>When I create an <code>RDS</code>, it seems to have an <code>inbound source</code> by default.</p>
  
  <p>For example, like <code>port: 5432, IP: 221.142.31.25/32</code>.</p>
  
  <p>As I understand, It means that the <code>security group</code> allows requests from the IP range(<code>221.142.31.25/32</code>) to access the port(<code>5432</code>).</p>
  
  <p>Somehow It seems to allow my local to access the <code>RDS</code> as well without any additional <code>inbound source</code> representing my local.</p>
  
  <p>In contrast to my local, when I try to have access to the <code>RDS</code> from <code>Lambda</code>, I have had to add <code>inbound source</code> <code>0.0.0.0/0</code>, otherwise, the <code>Lambda</code> has returned timeout error.</p>
  
  <p>My question is...</p>
  
  <ol>
  <li>What does the IP range(<code>221.142.31.25/32</code>) mean?</li>
  <li>How it allows my local to access the <code>RDS</code>?</li>
  <li>Why it denies <code>Lambda</code> but my local?</li>
  </ol>