Debian Configuration :: Routing - Forward All Traffic To An Ip To Another Ip?

I need to be able to do the following: Physical Router located at 192.168.40.1 On Ubuntu 10.04 Lucid machine:

eth0 with static ip 192.168.40.2
eth1 with static ip 192.168.40.3
eth2 with static ip 192.168.40.4

Associate a virtual address to eth1 with an entirely different network address such as 192.168.50.1 Do the same (virtual address) for eth2 -- e.g. 192.168.60.1 In the application:

register phone number A at 192.168.40.1 (The application will automatically use eth0 for this)
register phone number B at 192.168.50.1
register phone number C at 192.168.60.1

Somehow forward all traffic (including the register request) sent to 192.168.50.1 to 192.168.40.1 as if the register had been made directly to 192.168.40.1. In other words, the app "sends" registration and traffic to 192.168.50.1 but then Ubuntu forwards it to 192.168.40.1 (but the app does not know that). Similarly, forward all traffic sent to 192.168.60.1 to the router at 192.168.40.1.

Do the same for the reverse, forward all traffic that the router sends back to 192.168.40.3 (eth1) to 192.168.50.1 (within the Ubuntu machine) so that the app knows it is for phone B. Similarly forward all traffic that the router sends back to 192.168.40.4 (eth2) to 192.168.60.1 so that the app knows it is for phone C. Thus, the application believes that it is registering at 3 completely separate routers on 3 completely separate networks via 3 separate network interfaces but in fact is really registering all three to the same router (but does not know that). Similarly, the router believes that it is receiving 3 separate registrations because it receives each registration request and traffic from 3 separate interfaces and thus 3 separate mac addresses (i.e., of eth0, eth1, and eth2). Traffic sent to and from the router for each of the 3 phone numbers (via eth0, eth1, and eth2) are not mixed because the translation happens in both directions.