title: The uniqueness of MAC addresses
body: <p>Looking at <a href="https://stackoverflow.com/questions/7244336/get-something-unique-about-a-computer-in-order-to-create-a-free-trial">this related SO question</a>, I can't help but wonder about the uniqueness of MAC addresses.</p>
  
  <p><strong>How unique are MAC addresses?</strong></p>
  
  <p>I'm using them to <strong>semi-uniquely identify users.</strong> I have a website that users of virtually any device (PC, Mac, iPhone, Android phone, etc.) and any OS can hit via an HTTP request. I use a combination of IP address and MAC address to identify unique users.</p>
  
  <p>I assume the following cases can exist:</p>
  
  <ul>
  <li>A device has no MAC address (unlikely, sure, but anyway)</li>
  <li>A device has a unique MAC address</li>
  <li>A device has multiple unique MAC addresses</li>
  <li>Two or more devices have the same MAC address</li>
  </ul>
  
  <p>The first three of these cases are unique (the third because I only need a single unique MAC address). For the fourth case, how likely is this?</p>
  
  <p>That is: given 100 random users (perhaps Windows users for any Windows OS), how many of them can I expect to have the same MAC address? Is it just generally because of the limited length of MAC addresses? Or is it dependent on some sort of purposeful configuration change (MAC address spoofing)?</p>
  
  <p>I'm okay with MAC addresses being semi-unique, I just want some clarity on how to interpret the data.</p>
  
  <p>(I'm using the C# code against .NET 2.0 in the linked question against .NET 2.0.)</p>