more questions about firewall configuration

[Federico Guerrini <guerrini () elzira unipv it>]

Hi,
        this is my first time as a writer on this mailing list.
I have read carefully the discussion about "firewall configuration" because I 
want to set up a firewalling Linux Box. My problem is quite similar to the one 
presented in those messages, but there are some differences:

       ----------|
 internal net    |-----<my firewall>------external net-----<gateway>
       ----------|
                 
The IP addresses on the internal and external nets belong to the same C-class 
net (the gateway doesn't know to route packets to me through my firewall, but I 
learned from the messages I've read that this problem can be solved by setting 
up an arp proxy server on the firewall).
The problem here is that my internal hosts' official IP addresses aren't 
contiguos (i.e. my host numbers are .86, .163, etc. but some addresses in the 
interval [87,162] are not mine). Is there a solution? If yes, how should I set 
the netmask values for the network cards on my firewall and on my internal 
hosts? 

Thanks in advance for your help.
                                                                Federico