I actually do this now. I have an OBSD 4.3 box NAT'ing my desktop to my ADSL router using a single NIC. The OBSD box and the desktop talk to each other via the 4 port switch built into the ADSL router (which is also how the OBSD box talks to the ADSL router.) In my production environment at work I would never recommend this. However, this is my home, and the double traffic isn't detrimental at such low levels... just cuts down the effective bandwidth that can be utilized in half.
My ADSL router LAN address is 192.168.1.254. My OBSD box is 192.168.1.5 and 10.0.0.1 (with default gateway of 192.168.1.254). My desktop is 10.0.0.66 (with default gateway of 10.0.0.1), and the NAT address it uses is 192.168.1.66.
Here's my OBSD configuration-
hostname.rl0
Code:
inet 192.168.1.5 255.255.255.0 NONE
inet alias 10.0.0.1 255.255.255.0 10.0.0.255
inet alias 192.168.1.66 255.255.255.0 NONE
pf.conf
Code:
ext_if="rl0"
desk1_ext = "192.168.1.66"
desk1_int = "10.0.0.66"
binat on $ext_if from $desk1_int to any -> $desk1_ext
With this setup I use Snort (with BASE) as an IDS. It works nicely!