Networking :: Iptables Script - Define A Dynamic IPaddress?

in my iptables script I define the environment like this:

Code:

# ENVIRONMENT
# Private interface
IF_PRV=eth1

[code].....

if I give the IF_PUB interface the DYNAMIC IPaddress assigned by my ISP how can I define it in IP_PUB and also the NET_PUB so I can then use it for the firewall rules.

exemple rule:

Quote:

iptables -t nat -A POSTROUTING -s $NET_PRV -o $IF_PUB -j SNAT --to $IP_PUB