No not needed, ISPConfig uses iptables you can see what you need to install if you follow one of the how-to's on this page: http://www.ispconfig.com/documentation.htm
I have installed ubuntu 9.04 and enabled NAT / Internet Sharing. It works with out any problems. After I installed ISPConfig3, NAT / Internet Sharing no longer works. What should I do?
You can not use the ispconfig firewall on a server were you use internet sharing. Just let the firewall deactivated.
