As far as I can tell you can’t forward ports for individual ip addresses.
I would like to do this as well.
What you can do is set up a route for the servers using public IPs as above.
However on my network I have some servers that will never use certian ports. For example I have a web server that will never run VoIP, so if I could forward the ports for VoIP from that IP address to another server I could use the limited number of IP addresses more efficiently.
Another huge limitation I have found on this software is that there is no way to easily forward a block of ports such as 5060-5070 or 10000-20000. The first block wouldn’t be so bad considering it is only 10 rules, but there is no way I’m going to sit and forward 10,000 ports one at a time.
This is both a feature request and a realization that I need to go back to my previous firewall application until zeroshell is ready. I really like the software but it seems it is not ready for use in my situation.