Newbie firewall

[Nicolas]

Hello.
I want to use FreeBSD 5.2 on a standalone deskto I am trying to put up a 
firewall. For now i have been trying to use the client version in 
rc.firewall. I have a LAN connection with dynamic ip adress. 
How do I get the rc.firewall to know that I have dynamic adresses?
It worked yesterday but when I turned my computer on today the ip adress had 
changed. The only changes I made in rc.firewall client was to put in my 
net:
mask:
ip:
I made some changes also to rc.conf. It looks like this:

firewall_enable="YES"
firewall_script="/etc/rc.firewall"
firewall_type="client"
hostname="Moak.prisma.fbsd"
ifconfig_sis0="DHCP"
kern_securelevel_enable="NO"
keymap="swedish.cp850"
linux_enable="YES"
lpd_enable="YES"
moused_enable="YES"
named_enable="YES"
network_interfaces="auto"
nfs_reserved_port_only="YES"
router_enable="NO"
sendmail_enable="YES"
sendmail_flags="-bd"
sendmail_outbound_enable="NO"
sendmail_submit_enable="NO"
sendmail_msp_queue_enable="NO"
sshd_enable="YES"
syslogd_enable="YES"
usbd_enable="YES"

Hope that somebody wants to waste some time on my question.
Many thanks Nicolas.