configure firewall in etch

[leo <leonardo@softel.cu>]

hello,

is there any problem configuring a firewall in an dhcp client machine
when an IP address change every time I reboot. In the firewall rules I
denied (DROP) all incoming and OUTCOMING packet messages and after that
this line:

iptables -A INPUT -s 192.168.16.118 -j ACCEPT
iptables -A OUTPUT -d 192.168.16.118 -j ACCEPT

lets my IP address can do anything although in the next reboot this IP
changes so that line will not be valid anymore

my idea to fix this is copying this IP to an enviroment variable (with
export command?) to use it in my firewall rules script taking it's value
for example: 

iptables -A INPUT -s $myIP -j ACCEPT
iptables -A OUTPUT -d $myIP -j ACCEPT

the part I don't know is how can I copy the IP address from boot