Dynamic ipchains?

Hi! I've got a problem with ipchains. I have T-DSL, so I'm assigned a different ip-address each time I connect to my provider. However, to block ports via ipchains I need to enter my dynamic IP as source/destination, so for dynamic IPs I got to have a dynamic script, doesnt I ?!? I thought about picking my ip from ifconfig by a command similar to that: ifconfig | grep P-t-P | awk '{print $2}' (I haven't really worked it out yet!) and then using this for a shell-script, that prints ipchains-commands directly into the shell. Greets, Jan!