IPs changing

[Roberto Arcomano]

Hi all,
I've 2 classes of IP addresses: X and Y, only the X one is phisically 
assigned to real machines, while Y one is "virtually" assigned. My purpose is 
the following:
1-) When a X(i) host requested for an address, its IP address (X-like) would 
be changed to the "virtual" IP Y(i), Y-like address (same thing when the 
packets back to source!)
2-) When an Internet (from outside) host made a request to Y(i) host 
(virtual), the packet would be go to X(i) host (the same for the answering 
packet which back to the Internet host).

As a matter of fact I want to exchange 2 (or more) IP addresses without side 
effects (tipically made by MASQUERING, when it is requered special Linux 
modules to use some services like chat, real audio and so on). So I would 
like to have real ip forwarding (with IP changing).
Thank you very much for your help and your great support.
Best Regards
Roberto Arcomano