[TriLUG] Drop and insert transparent firewall (OpenBSD)

[Jon Carnes]

Jason Tower showed me a neat trick the other day - using OpenBSD to
insert a Firewall/packet filter transparently into an existing network.

The firewall uses no ip addresses and sits between the router and the
companies external switch.  The external switch has various boxen
attached - each of which uses an external IP address. All the external
IP addresses are in use, so the firewall/packet filter had to be
inserted without using any additional IP's.

This does the trick rather nicely:
http://www.openbsd.org/faq/faq6.html#Bridge

Enjoy!