http://blogs.msdn.com/b/virtual_pc_guy/archive/2007/09/17/how-to-enable-duplicate-mac-addresses-on-virtual-server-2005r2.aspxJust wanted to pass this one a long, is embarrassingly enough I actually learned about this on one of our public newsgroups. By default Virtual Server 2005R2 will detect if you have virtual machines on the same box configured with duplicate MAC addressesen-USTelligent Evolution Platform Developer Build (Build: 5.6.50428.7875)http://blogs.msdn.com/b/virtual_pc_guy/archive/2007/09/17/how-to-enable-duplicate-mac-addresses-on-virtual-server-2005r2.aspx#5360154Mon, 08 Oct 2007 14:12:20 GMT91d46819-8472-40ad-a661-2c78acb4018c:5360154saxa<p>Well, you can configure more than one virtual network...</p> <p>I can imagine the situation where you have some VM in a production, which is connected to &quot;External Network&quot;. Now, you want to accomplish some tests. You make a copy of the VM in question and connect it to a closed &quot;internal&quot; virtual network... You will want to keep the MAC address.</p> <div style="clear:both;"></div><img src="http://blogs.msdn.com/aggbug.aspx?PostID=5360154" width="1" height="1">http://blogs.msdn.com/b/virtual_pc_guy/archive/2007/09/17/how-to-enable-duplicate-mac-addresses-on-virtual-server-2005r2.aspx#5114462Tue, 25 Sep 2007 10:18:24 GMT91d46819-8472-40ad-a661-2c78acb4018c:5114462Todd<p>So... this just begs the question, why might one want to run two (or more) VM's with the same MAC address?</p> <div style="clear:both;"></div><img src="http://blogs.msdn.com/aggbug.aspx?PostID=5114462" width="1" height="1">