PoPPaP's Blog

Setting Parallels desktop to use NAT on certain network that multiple MAC address on the same interface can cause problem.

From: macosxhints.com

Go into Parallels’ network configuration section, and tell it to bind to en0. Then go into the Sharing pane in OS X’s System Preferences and tell it to share from AirPort to Ethernet. This has the effect of setting up NAT to your en0 interface, and Parallels hooks into that to get internet access. Since everything is masqueraded through your airport (en1) interface, it magically works.

Update: Newer Parallels Desktop has option to select “Shared Networking” and Parallels will do the NAT for you.