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Spoofing an IP

Last Modified: 2010-04-10
Here's the problem.. We have a site-to-site VPN connection with our parent company's network in England.  Over the past weekend, we changed our internal IP address scheme from a /24 to a /23 network to accommodate an influx of new employees and to prevent conflicts with our internal default router and the default router of our VPN clients.  In doing so, we picked an IP range that was already in use by our counterparts in England.  They have another remote site that uses the network as well, which creates a conflict when we attempt to connect our site-to-site VPN.  Is there any way to create static routes so that our England counterparts sees our network as a 192.168.x.x?

Here's how traffic flows at our site:

12.xxx.xxx.8(External IP address of our PIX firewall) -> IP address of our PIX) -> IP address of our Cisco 2811 Router) -> IP address of our Cisco 2811 Router, which is the default route of our network)

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No. I would say, change your ip address.

Another workaround would be, putting a router between the LAN and VPN router. Like this

EnglandVPN router-----------VPN router-----------------192.198.x.xROUTER10.10.0.0/23------------------------LAN10.10.0.0/23
You have to make sure that you are using only static routing. No routing protocol.

Change IP's.  Pick something else like:

Les MooreSystems Architect
Top Expert 2008

Depending on your VPN endpoint devices, you might be able to nat between the two networks and work just fine.
I know it's possible using Cisco PIX as endpoints.
It's natting, not routing that you need.


On my end is a Cisco PIX 506e, England is using a Checkpoint VPN-1.

Do you have a sample config for this that I could use as a basis?

Who is in charge of the network as a whole?  Your IP range should be assigned by that person so as to avoid conflicts.  What other subnets are in use?
Systems Architect
Top Expert 2008
This one is on us!
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Steve JenningsSr Manager Cloud Networking Ops

. . . make sure you don't NAT the addresses to the same IP range! (Har!!) Sorry. Couldn't resist.

As RDAdams said, implement the ip address as assigned by whoever has done all the subnetting assignments.
IP addresses MUST be unique within the broadcast space that is utilized.
You really dont have a choice and there aren't any passable options as a work arround.

The short version of the story is that you should not have implemented the 10.x.x.x addresses without verifying they were not in use and now they must be corrected.
If there is not currently a central authority on your IP network for ip designations, then you should assume that responsibility.
Determine what subnets are in use and create documentation for the subnets use and where they are in use.
Distribute the documentation to all networking departments in any of the companies that are or will ever be in use at any of the companies that connect to any of the networks used by the companies (Directly - if there behind NAT, thats fine).

This should most likely be a simple list such as:
10.10.0-4.x england
10.10.5.x your company
10.10.6-254.x unused

Also, I would not implement knightrider2k2's suggestion in a production environment.  You do not want to have any ip addresses duplicated at any point in your network.


Worked perfectly, lrmoore.  Thanks a ton :)  If we used the VPN for anything more than simple video conferencing and intranet traffic I would be more worried about this.  I've since taken control of the networking scheme for both the US and the England sides of our network so we will not run into this again.

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