This is quite a handy app for creating small virtual routers. See the website for more detail… The commandline is similar to cisco – so its good for practice if you are going for your CCNA.
http://www.vyatta.com/products/virtualized.php
Oh – its open source and free!
I’m a bit lazy in most cases and just add a few virtual nics into my VM’s and do the routing though them. But some people may find this handy for a single point to manage networking.