I need to set up a firewall and DNS server on a single computer HELP PLEASE

[kla]

I need to set up a firewall and a DNS server on a single computer and my problem is that i don't understand how to configure network cards for that. In fact, i don't know why there must be TWO network cards for that. All that i've done by now is to install OpenBSD, set up firewall rules, make firewall and DNS start after every rebooting and enable packet forwarding. But i don't know how to make it work like firewall and DNS should for small network of ten to twelve computers that must be connected to internet. My biggest problem is that i can't find understandable answer in any OpenBSD manual that is available to me.

So my main question would be why do i need to configure two cards. After that, how i configure them. I understand that for dynamic IP adress assignment i can configure in localhost.if

dhcp NONE NONE NONE

But one manual that i've seen states that in order to configure network card one must write following:

inet IP-adress 255.255.255.0 NONE

And it doesn't mention wether it's for static or dynamic assignment of IP adress.

Can someone please help me?

P.S. Sorry for my bad english.