title: Using substring of a GUID for ID, all GUIDs generated on same machine with Asp.NET
body: <p>I've read a <a href="https://stackoverflow.com/questions/1801961/using-parts-of-guid-as-id">similar question</a> and <a href="http://blogs.msdn.com/b/oldnewthing/archive/2008/06/27/8659071.aspx" rel="nofollow noreferrer">Raymond Chan's blog post</a>, but still have a question around using a shortened GUID as an id. I understand that GUIDs are unique, but substrings of GUIDs are not.</p>
  
  <p>I am building a site that supplies the user with a reference number for an order. The client does not want to use an auto-incremented Int, as this will expose the total number of orders. They also consider GUIDs too long and difficult to repeat over the phone.</p>
  
  <p>However, as we will only have one web server, it has been suggested that we can use the non-MAC address part of a GUID, to give a shorter, unique identifier.</p>
  
  <p>My understanding is that the current GUID algorithm does not allow for this. Is this correct? Can GUIDs created on the same machine be shortened yet still unique?</p>