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For example, let's say you're bridging network 'a', which uses addresses through, and network 'b', which uses addresses through 

When a machine on network 'a' wants to send a packet, it has to make a
decision about that packet: is it local or not? The way this decision is
made is really simple: part of the destination IP address is compared to
part of the source IP address.

The part of the addresses that is compared is determined by the subnet mask.
Any bit that is turned on in the subnet mask is compared. (I'll come back to

So if the part of the source and destination addresses that is compared
matches (Sending from to, with a subnet mask of, then the sending machine knows that the destination is
local, and sends the packet out unmodified. 

If the compared part doesn't match, the packet is not local, and is sent to
the default gateway to be forwarded. So a source of and a
destination of will go through the default gateway to be

This addressing problem is separate from forwarding broadcast traffic.
Depending on the type of broadcast traffic, there are different ways to
handle it. If it is Windows/SMB/CIFS share names, there is a way to
configure WINS servers on each side of the tunnel to relay broadcasts to
each other. Other broadcasts have other solutions. 

-----Original Message-----
From: Adrian Snyman [mailto:adrians,AT,datrix,DOT,co,DOT,za
Sent: Thursday, April 26, 2001 3:19 AM
To: ekool,AT,ns1,DOT,netmdc,DOT,com
Cc: cipe-l,AT,inka,DOT,de
Subject: Re: Bridging two networks, need broadcast

On Wed, 25 Apr 2001 ekool,AT,ns1,DOT,netmdc,DOT,com wrote:

> I need to bridge two remote networks together, and broadcast traffic has
> to be forwarded.
> I have a linux box located on the network with an eth0 and an eth0:1
> virtual interface. The internet routable IP is eth0 and eth0:1 has an ip
> in the 192.168.1.X range.
> I have an extra linux box here on my network and need some windows boxes
> to be able to have IP addresses in the 192.168.1.X range, and broadcast
> traffic needs to work between them.
> (please, dont ask why, long story)
> Now, i dont quite understand the documentation. Does the cpidb0 device
> need to have the same IP as the 192.168.1.X eth device for it to forward
> packets across the link properly?
> I have gotten it setup so i can ping (which i assigned as
> the cpidb0 device on one end) from (which i assigned as the
> cpidb0 device on the other end) -- but no matter what route statements i
> use, i cant get beyond that. Which, im sure i'm doing something wrong
> since doing an add route -net would require me to have two gateways.
> Please, any advice is appreciated.

Remember, the VPN you are creating is effectively another network ..

So, create the interfaces using:

CIPE Interface 1:
CIPE Interface 2:

now you should be able to ping the remote interface ..

Then set up the routes :

(CIPE 1) route add -net gw
(CIPE 2) route add -net gw

I'm not to sure if this is what you want (or if it even works !!)

Then again, you could just use different ip ranges on either side of the
VPN ..

|    .--.    Adrian Snyman - Datrix Solutions                     |
|   |o_o |   mailto:adriansREMOVE,AT,datrix,DOT,co,DOT,za                    |
|   |:_/ |   Tel: +27 11 888-1869                                 |
|  //   \ \  Cell: +27 82 600-1211                                |
| (|     | )                                                      |
|/'\_   _/`\ Big Bang Theory Sense: First there was nothing ..    |
|\___)=(___/                        Then it exploded ..           |

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