A good opensource program to do this with is PacketFence 
http://www.packetfence.org. Its written by a couple of guys at Harvard. 
It can be used in a passive ARP spoofing mode or an inline mode. We use 
in in our dorms and it works great. The newest version has better VLAN 
support. Using this cut down on our P2P use also.

Jonathan

Tim Vaughan wrote:
>> anyway we can setup our firewall to do this??
>>
>> http://www.pcworld.com/resource/printable/article/0,aid,123138,00.asp
> 
> Not with m0n0wall alone.  This requires a managed switch, VLAN
> management software and a bunch of other software to detect infected
> machines.
>