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Two simple networks connected by two cisco 1711s. Both networks are using class c addresses. So... when data moves from one site to another and the destination IP is used at both sites will I have a conflict? And if so how do I setup NATTing to ensure it doesn't happen?
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That sort of depends...

Site A   LAN -->1711 ---- <?Internet?> ---- 1711<-- LAN   Site B
   192.168.2.x                                                             192.168.3.x

As long as site A and site B LAN's are in different IP subnets, there should not be a problem..
It all depends on what is connecting the two LAN's...
If both LAN's are the same IP subnet, then you have to use NAT at one end or the other.
I would need to see your config's to offer more detailed assistance..
>>destination IP is used at both sites will I have a conflict  YES


Network Address Translation allows a single device, such as a router, to act as agent between the Internet (or ""public network"") and a local (or ""private"") network
I assume you mean you're using the same private IP at both sites. For example, both sites might have a network with hosts with the IP Assuming these networks are connected to one another exclusively via the Internet (without a VPN or similar tunnelling technology) then you will not have a problem as the two hosts never see the internal IP of each other, they only see the IP's of the router for the other computer's network. However, you will have a problem if either these two network are connected directly together via their routers (e.g. using a direct WAN connection) or if you try to VPN from within one network to the other network. Therefore, it is best if you can do it to subnet the two networks separately. For example, setup one with and the other

Or you could just readdress one end into a unique address range to eliminate the overlap. I am not sure if this is doable
but it would make things easier in the long run. (just my 2 cents).   ;}

harbor235's advice is exactly right, but I too suggested this in my response. It is definitely best to keep two sites with any kind of relationship (technological or any other) subnetted differently. As there is no shortage of available private addressing you should not have a problem doing this.

Unless you have a very large network or your network is being used by a lot of users 24/365 then it shouldn't be too hard to switch. As a first step I would reduce your DHCP lease time to a very short time. Then wait at least 1/2 the current least time (e.g. if the current lease time is 7 days, wait at least 4 days) before switching. If you switch on a Friday night all hosts should get new IP's before anyone notices. If you could force a reboot  or "ipconfig /renew" for all hosts after the change that would be even better.
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