Interface NAT forwarding via iptables?

[thehemi]

I'm not quite sure how to even format the question, so let me
explain what I'm trying to accomplish and we'll go from there.

eth0 ("public")
eth1 ("private")

I need both of those interfaces to remain open and accessible.

eth0:1 (alias on "public")
eth1:1 (alias on "private") # this may or may not be needed

Step 1-

I want anything coming into eth0:1 (any port) simply redirected
to a specific host that's on the "private" network. This network
does not have "public" access, so it will need some kind of NAT
in order for the return packets to get through.

Step 2-

Ideally I would like to traffic from a "private" host that comes to
eth1:1 to get redirected, based on destination port, to a server
on the "public" network. Again, I assume some kind of NAT would
be required to get the packets back through.

Does this make any sense? I'm going through all sorts of iptables
documentation but I'm not sure anything that I'm finding really is
doing what I'm trying to accomplish.

[thehemi]

I'm not sure that iptables supports aliased interfaces?

[quitespy]

Iptables does not support Aliases, it sure would be easier to add a second NIC