title: What's different between URI, request-URI and URL?
body: <p>I'm learning Web tech and teacher cannot give me a satisfactory explanation.
  I'd like to give a few example, please help me to point out am I right.</p>
  
  <ul>
  <li>I know URL is a subset of URI</li>
  <li><a href="http://www.example.org:56789/a/b/c.txt?t=win&amp;s=chess#para5" rel="noreferrer">http://www.example.org:56789/a/b/c.txt?t=win&amp;s=chess#para5</a>
  is a URL and also a URI</li>
  <li>every address we type in browsers can be called URL</li>
  <li><a href="http://www.example.org:56789/" rel="noreferrer">http://www.example.org:56789/</a> is also a URI</li>
  <li>the part: <code>a/b/c.txt?t=win&amp;s=chess</code> is <strong>request-URI</strong></li>
  <li>the part: <code>para5</code> does not belong to the <strong>request-URI</strong> is just a fragment</li>
  </ul>