title: What's the advantage of symmetric NAT (as far as ISP is concerned)?
body: <p>After trying to connect two machines via WebRTC I learned that one of the machines is sitting behind a symmetric NAT (here's a great <a href="https://webrtchacks.com/symmetric-nat/" rel="noreferrer">article</a> about it by <a href="https://stackoverflow.com/users/4352772/philipp-hancke">Philipp Hancke</a>). 
  This machine hasn't been always sitting behind a symmetric NAT. I could connect to it a couple of months ago without any problems (using only STUN servers). The hardware and software on that machine hasn't change since then, so I figure that the ISP must have done something. </p>
  
  <p><strong>My questions are:</strong> </p>
  
  <ol>
  <li>What's the advantage of using symmetric NATS?</li>
  <li>Since using TURN servers is expensive, enabling symmetric NATs could be considered a hit on peer-to-peer connections, because it would prevent developers from utilizing WebRTC technology at all. </li>
  </ol>
  
  <p>What do you think? </p>