How can I bridge two wired ethernet interfaces on Mac OS X (e.g. the current MacPro comes with two ethernet ports)?

Googling turned up (e.g. this Apple forum post and this openvpn post) that this is fairly easy on Linux (using the brctl command) and under Windows (via Network Connections > right-click > Bridge Connections), but how is it done under Mac OS X?

BTW: There also doesn't seem to be a macport for brctl ('port search brctl' didn't turn up any results)

Note: I don't want to have 'internet sharing', which creates a new network (by handing out network addresses in a new range). I want to really 'bridge' two interfaces so to keep the same network subnet.