Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!samsung!raybed2!hlh
From: hlh@raybed2.msd.ray.com (HOWARD HANTMAN)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: SLIP and IP routing problem
Message-ID: <2165@raybed2.msd.ray.com>
Date: 16 Mar 91 00:11:55 GMT
References: <1991Mar11.190228.15146@mprgate.mpr.ca>
Organization: Raytheon Co., Tewksbury, Mass.
Lines: 48

In article <1991Mar11.190228.15146@mprgate.mpr.ca>, parker@mprgate.mpr.ca (Ross Parker) writes:
> I'm having a routing problem with the following configuration:
> 
> Currently, the Ottawa systems and the systems here are using the
> same IP address space - both in the class B address '134.87'. I can
> understand routing failing with this configuration... with only one
> network address, there's nothing to route *to*. I have, however,
> tried setting up the SLIP line with it's own address (we have a
> class C address at our disposal, and the route to Ottawa still isn't
> reachable.
> 
I believe your problem is that you can't have a disjoint IP network.
Putting a Class C address on the SLIP interfaces does not change the
fact that the hosts in Vancouver on network 134.87 are on (or SHOULD be on)
the same network as the hosts in Ottawa on network 134.87. I don't think
its legal to set up a route to network X from network X via network Y!
The easiest solution would probably be to use subnetting. From your diagram:
>          Ottawa                                 Vancouver
>         'mprott'                                'mprgate'
>     ------------------                     ------------------
>     |134.87.137.1    |                     |   134.87.131.12|
>  ___|______^         |                     |          ^_____|___
>     |      192.67.9.1|   Slip line         |192.67.9.13     |
>     |          ^_____|_____________________|______^         |
>     ------------------                     ------------------
> 
The third byte of the IP address for the two LANS is already different.
If that third byte is consistant at the two sites you can set the
network mask to 255.255.255.0 and then Ottawa and Vancouver will have
two separate IP networks legally linked by a third one. If due to number
of nodes, you cannot devote an entire byte to subnets, you may need to
look into changing whatever the smallest number of addresses is to get some
number of significant bits to be different and subnet on that.

A second solution, much clumsier but a last resort if there is no way
to change addresses to meet the above, is to use proxy arping. On each local
gateway, you would need to put a static route for each remote node you
wish to reach. You would then also need to add proxy arp entries for each
of the remote hosts. Local hosts would not require any routing information.
As far as they would see all nodes on their network (134.87) would be
directly reachable via ethernet, just as they should be.

					Howard Hantman
					Computing Consultant
					Software Development Center
					Raytheon Co.
					hlh@swlvx2.msd.ray.com
					...{linus,applicon,cg-atla}!raybed2!hlh