I've been playing with Parallels Desktop on OS X as a replacement for Virtual PC on Windows in terms of testing complex Windows network designs. The lack of "undo" disks is still a major problem, but I've found a way to build an internal, isolated network for your VMs (Virtual Machines) to play in.
Parallels, by default, offers two choices for VM networking -- Bridged or Host-only. With Bridged, the VM connects to your Mac's network and is basically on the local network. With Host-only, the VM is placed in an isolated network, as desired. But then Parallels does something annoying: It adds the Mac OS X machine to that isolated network and fires up a DHCP server. While it gives you the ability to adjust the DHCP scope, there is no handy way to turn it off. Killing OS X's Parallels network adapter just dumps the VMs back into Bridged mode.
If, like me, you need to test networks that include a DHCP server (for LDAP settings, NAP settings, etc.), this is a problem. You don't want your host OS X machine to be a DHCP server on your isolated network. Fortunately, this DHCP server is easy to spot in Activity Viewer (or top) -- it's called Prl_dhcpd. Killing this process stops the DHCP server, and lets you run your own DHCP server in the isolated network.
It'll come back on reboot, so kill it again before launching your test machines if you've rebooted since last use. This is what I do (since sometimes I do want it running for casual Parallels use). If you want to prevent it from running on boot, the file is (naturally) located in /Library -> StartupItems -> Parallels. Moving it out of there should stop the "Parallels DHCP server launches on boot" activity, but I haven't bothered to test that. You could also write an Automator action or shell script to kill this process without using Activity Viewer, but I haven't bothered to do that, either :).
Hopefully Parallels will include a nice preferences option to do this in a future update -- I was quite disappointed to discover the DHCP tab had a way to change the scope, but no "turn off" button.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061102124948456