[CentOS] iptables routing question when using multiple IP addresses

I've been searching around for a couple of days, and I just can't seem to find the answer I'm looking for.


I have a 6.x box that I use as my gateway firewall.  It has three NICs; 1 external, 1 internal, 1 for a guest network.


I have various inbound traffic routed to separate internal machines based on the application (ie. http, smtp, ftp, etc....).


All works well, but I'd like to add a second IP address to the external NIC and then route a second https stream to a different internal machine.


So https traffic on one IP address goes to one internal machine and https traffic on a second IP address goes to a different internal machine.


Seems like it should be an everyday configuration, but I can't find any clear examples on how to do it.


Could someone please point me in the right direction?


Thanks,


Ken Gramm