title: Where do UUID namespaces come from?
body: <p>The <a href="https://www.rfc-editor.org/rfc/rfc4122.html#section-4.3" rel="nofollow noreferrer">UUID specification</a> defines 4 predefined namespaces which it describes as &quot;potentially interesting&quot; - meaning among other things, &quot;if other people have generated UUIDs in this namespace you can verify them&quot;:</p>
  <ul>
  <li><code>6ba7b810-9dad-11d1-80b4-00c04fd430c8</code> for DNS</li>
  <li><code>6ba7b811-9dad-11d1-80b4-00c04fd430c8</code> for URL</li>
  <li><code>6ba7b812-9dad-11d1-80b4-00c04fd430c8</code> for ISO OID</li>
  <li><code>6ba7b814-9dad-11d1-80b4-00c04fd430c8</code> for X.500 DN</li>
  </ul>
  <p>Where did these come from?</p>
  <p>Specifically;</p>
  <ul>
  <li>If I'm generating my own namespace UUID do I need to avoid anything in particular?</li>
  <li>I'm aware how big the UUID space is, but does this have any implication on collisions?</li>
  <li>Why have they chosen the 4th octet to increase as a kind of UUID 'version number'?</li>
  <li>Do my questions imply that I'm missing something fundamental about UUIDs?</li>
  </ul>