DaemonNews.org has the tutorial: "With OpenBSD and IP Filter, a bridge can be setup that filters incoming traffic. The bridge is not assigned an IP address on either
network card. The benefit of this type of firewall is that the sender of an incoming packet is entirely oblivious to the existence of an
intermediate bridge. This provides transparency and allows our firewall, which we maintain on the bridge, to securely filter
incoming packets and forward them without modification to the internal destination. This guide walks through two typical home
network connection setups (ADSL and cable modem) using OpenBSD 2.8."
March 1, 2001