Okay, I am only talking about the special case where you are having problems. I am trying to understand it, and I will explain to you what I understand the problem to be. I need you to let me know if I am correct in my understandings.
You mentioned " This requires that the address assigned to me in the PPP-connection is in the client's network range. NT-RAS does this and our ISP also.
In this special case things unfortunately are different. There is a router at the other end and this router unes a seperate, internal network for connections. So I get assigned 220.127.116.11, which becomes the address for masquerading. This is of no use, since the guys at the other end would have to add routes to this address on every machine.
The router at the other end has an ethernet address of 18.104.22.168
Can NAT be of any help in this situation? I.e. 'fake' the masquerading address to 22.214.171.124 ? "
are you saying that you do an ISDN dial up from your linux box to their router, which hands your end of the isdn link 126.96.36.199, and their end of the isdn link is 188.8.131.52, and the ethernet of their router is 184.108.40.206, and you need to get to the 91 network from your 10 network?????
If I am correct.....
From your 10.1.1.x, with a default route of 10.1.1.1 (the machine which is doing the ISDN dialing), you telnet/ssh/whatever to 91.x.x.x, and your box knows (because you have a static route pointing out your isdn interface to 91.x.x.x) to kick your packets along to 220.127.116.11. When their router gets the packet, it sends it to the 91.0.10.x box where it belongs. But that is not your problem. Your problem is how their workstations are supposed to know that they have to go through 18.104.22.168 to get to 10.1.1.x, right? I can think of a couple of ways. First, the easy way. All default routes on all boxes know that 22.214.171.124 is where they send everything that they don't know about. Second, in case you already have different default routes set up for a million workstations, put a route in _their_ default router to say "all 10.1.1.x traffic has a next hop of 126.96.36.199". Thirdly, in case either their default route or all of their workstations already know how to get to 188.8.131.52 (as they should, since *their* box is handing out the IP address!), have your isdn dialup machine do masquerading from your 10.1.1.x network across the ISDN link. This way, all of them over there will see your packets comnig from 184.108.40.206.
I am interested in knowing how this turns out for you.
have fun, rob