What exactly is ipfilter?

[Emmanuel Gravel]

I'm looking through rc.conf and the kernel config file for FreeBSD 4.9
(recently downloaded it, my last upgrade was 4.5 so I was way behind,
and this is a new install because my old firewall died). I'm used to
using ipfw and natd for my firewall, but now I'm seeing ipfilter, ipnat
and ipmon. I've done a google search on all of www.freebsd.org for
ipfilter, but it only seems to show up in release notes, and the online
handbook doesn't really talk about it. Since I haven't recompiled my new
kernel, should I consider this instead of ipfw and natd? What's the
difference, exactly?

On a related note, I'm not sure what the usefulness of IPDIVERT is
either, so I don't know if I should compile it in the kernel or not.

Thanks!