Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

Connecting overlapping networks using NAT

Hi all,

My problem is the following:

I have a network A using addresses and

a network B, which also uses

1. Readdressing is not possible, but connectivity must be provided.

2. Due to the large amount of possible IP addresses only dynamic NAT should be considered. Ideal would be to use NAT with host number preservation.

3. It must be possible to establish communication from both networks A to B and B to A.

4. The usage of a DNS server for forcing an outside NAT table entry (as described in the Tech Note "Using NAT in Overlapping Networks" is not posssible.

Question: Is there a way to accomplish the above stated task?

Thanks for comments in advance, Peter

New Member

Re: Connecting overlapping networks using NAT


Can you be more specific and tell us what is the scenario? How many routers are involved and how?


New Member

Re: Connecting overlapping networks using NAT

Hi Peter,

As Nuno indicated, we need to know a little more about your network (physical connectitvity between them, routers, f/w).

But, as you probably have incompatiblity about IP addressing between both network, for sure you need to NAT, both side. It's the easy part. After, to reach servers or hosts from any site, you should have translation as done by PIX f/w with static statement.

But we need more info to complete the job.



New Member

Re: Connecting overlapping networks using NAT

Hi guys, thanks for your reply so far, although I'm already at home, I've created a small sketch to illustrate the problem. Please take a look at:

To summarize again:

Three things are important:

- All NAT should be done dynamically

- Communication should be established from both sides

- I have absolutely identic network addresses on both sides

Thanks in advance


New Member

Re: Connecting overlapping networks using NAT


Unfortunately I don't see a solution for this case...

If you don't have DNS there's no way the router builds its NAT table appropriately.

The solution I thought doesn't work either. I thought you could do NAT on both routers (ip nat inside source...), but you say you can't. Even if you could it would only work after each host on either side tried to communicate at least one time with some other host beyond the router on each side.




CreatePlease to create content