Proxy ARP

[wsuetholz@centonline.com]

Hello, 
  I need to put a debian box between an cheap ISDN router that our ISP provided
and our network to provide firewalling and some address translation services. 
We have our own class C, and would like to set it up so that this new box says
that it knows how to get to the ISP, without changing the ISDN router config.
The ISDN router does some filtering currently, but I'd like to get a little more
flexibility.

Current config:
  ISP - ISDN Router (.60) - rest of network.

New Config:
  ISP - ISDN Router (.60) - Debian (.64) - rest of network
OR
  ISP - ISDN Router (.253) - (.254) Debian (.60,.64) - rest of network

  I have two network cards in the Debian, and want to have the Debian say that
it is .60 as well as being .64, and also that it needs to send traffic out the
second network card that is destined for .60 or anywheres else for that matter.
I'd like the ISDN router to still think that it is on the rest of network, and
have the Debian handle things transparently.

Does anybody have any pointers?  I tried to set up proxy arp, but I guess I
don't understand how.  I almost would like the second option using the .254 and
.253 addresses, because then I can use a .252 subnet.  I would like to make
this seamless to the rest of the network.

Thanks for any help
Bill Suetholz