Denys incoming via en0 (you prolly don't want this since you only have one card), I take that back.. You will want it to deny stuff coming in from your ppp connection (im gessing you use an adsl modem or somthing using pppoe.

So yea, replace all instances of en0 with your outgoing connection

That surprises _me_.

[~]$ sudo ifconfig en0 inet 192.168.0.1 alias

will create that ip address on the en0 device, which can be accessed from your LAN if your other machines are set up to look for it, and if you've enabled "internet sharing" with natd and ipfw (along with some other useful firewall rules, naturally).

Setting up your firewall/NAT rules manually can be instructive, if somewhat mind-numbing.

There's an interesting shareware program for OS X called Firewalker X2 that works nicely with ipfw to create a very solid firewall. <http://www.pliris-soft.com/>

Warren.

get yourself a router. it has a hardware firewall to the outside, and you can do what you like on the inside.