Get a router that is compatible with DD-WRT. This feature is built in.

Between VirtualBox (www.virtualbox.org) and Vyatta (www.vyatta.com) you can pretty much (si/e)mulate anything networking. VirtualBox is of course virtualization software and Vyatta is an x86 open routing platform. Vyatta's command syntax is very similar to CISCO and anything you can do with a CISCO router (and to some extent, CISCO's security devices...) you can do with Vyatta. Vyatta is a custom linux distro with full blown routing capabilities. It can also serve as a client if needed. I often have to simulate networks in my line of work and Vyatta + any virtualization platform is pretty awesome at making things work. Vyatta's memory and disk requirements are pretty low and for what it's worth, you could likely even run a few Vyatta virtuals on an x86 based netbook without being too incredibly slow. About being able to tweak things on the fly, I would create an additional interface on each Vyatta box and set up a host based management network and then SSH configs with the different settings I felt necessary to explain what I was teaching. Script it, then you look like the master of your domain =) Good luck!