Merging Networks

Unanswered Question
Nov 1st, 2007

Is there other possible way to connect another network from a remote site with conflicting ip subnets with the local network without using nat?

I have this problem too.
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Jon Marshall Thu, 11/01/2007 - 15:16


If you want to be able to route between them then you have 2 choices really

1) Readdress one of the subnets.

2) Use NAT

The problem is simply

Network A host A1 =

Network B host B1 =

A1 wants to communicate with B1. Again 2 outcomes really

1) exists on Network A. In which case A1 talks to the wrong host.

2) is not used on Network A but A1 will just arp out locally for B1 because it thinks it is on the same network.

Either way it just won't work unfortunately.


rsagustin Fri, 11/02/2007 - 14:51

Thanks Jon,

Although it did not resolve the issue, i appreciate for taking the time to answere my concern. It would be impossible for the remote site because it is a large network with various subnets, and also impossible on the local side as well. We are connecting this network thru MPLS. I'm just wondering how other large organization merge their networks without changing their IP addressings . Again thanks for your reply.

cisco_lad2004 Sat, 11/03/2007 - 01:00

Hi there

With large organisations, public IP addresses are often used for servers and main access devices therefor unique.

also, most of the time end users always connect to these servers, FWs, mail, DNS etc...but hardly directly. so one PC does not talk to another.

when merging is a requirement, MPLS VPN could be used to allow for 2 or more different organizations to use overlapping IPs as long as they communicate through servers with Public addressing. FWs are often the separator, where each VPN, maps to an interface.

otherwise, Jon's reply holds 100%.



rsagustin Mon, 11/05/2007 - 06:53

Thanks Sam,

Both you and Jon had great answeres regarding the issue. I'm not sure if i'll run on the same problem if just one of the router link on the remote site has an IP conflict with the local link between routers, as shown below. Thanks.







paul.matthews Mon, 11/05/2007 - 10:09

How do organisations merge without changing addressing? basically they don't. When two networks merge, ultimately something will need readressing, it is more how do they manage?

The first bit will probably be opening a VPN tunnel to allow access to the main corporate web server, and jining up email - cross publication of contact info, that sort of thing. At that point it is two totally separate networks that talk to each other.

Ideally major systems would be on public addressing, so that no NAT would be needed.

Actually merging the two networks may or may not actually happen. It is rare that there is actually a merger - normally it is a takeover. What may happen is that office by office, the "minor" company gets migrated to the major companies network. With sensible planning, the email services etc should already be available, and all that happens is a change of network address, and logical position within the network.

MPLY may be an option, as long as datacentre addressing is sortable, with route targets to handle routing, and maybe NAT for source addresses as you hit the datacentre.

I would hate the job of merging two large networks, especially if both used different vendor's kit. Imagine the fun if someone the size of HP running a Cisco network was to merge with someone the size of Sun running Juniper!


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