a MAC user is pursuing MS certs, hehe, not bad. :)
as per your requirements, you may choose HOST ONLY in Parallels's network configuration. it will give you a separated VIRTUAL network for your lab environment. your physical home LAN and the virtual LAN on MAC can't access each other, though all VMs on the virtual LAN can access each other like they are living on a real network.
FYI - Parallels Desktop Networking FAQ and Troubleshooting Guide
http://forum.parallels.com/thread1877.htmlhope it helps,
bbao