NAT issue

Unanswered Question
Jul 29th, 2009

I have an issue where we have a private network, This traffic NATs to a single DHCP assigned address on the outside interface of Cisco 1841. The 1841 can be connected to our netowrk or it can be connected to an external ISP. When in station, the IP address the 1841 receives is a routable IP address and thus we only have to NAT once. to routable address.

However, when connected to the external ISPs, a NAT has to be performed at the 1841 and then again at the cable/DSL modem. How do I configure my outside interface to be able to handle this extra occurance of natting? Currently, we are able to ping from a laptop on the inside network to the address on the backside of a CABLE/DSL Modem. If we put the DNS address of, we can actually resolve names using the external ISP. But we cannot surf the web or ping open sites like

Any help would be appreciated.


I have this problem too.
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Paolo Bevilacqua Wed, 07/29/2009 - 07:59


no ip route FastEthernet0/1

dhcp will assign a default gw, the command above only makes trouble.

Configure the cable/dsl modem for bridging and you will have a single NAT only.

Even better, get an appropriate interface for the router itself.

dpatkins Wed, 07/29/2009 - 08:32

If you take out the default gateway, how does it know to go out the fa0/1 for unknown traffic?

dpatkins Wed, 07/29/2009 - 12:22

I took out the statement ip route FastEthernet0/1 and it would not work. From the router, I can do an nslookup to have it resolve. From the desktop, it will not find the Cable modem as the DNS server.

How do I ensure that the DHCP service uses the server from the external interface source?

Thank you

Paolo Bevilacqua Wed, 07/29/2009 - 13:08

dhcp is one thing, dns is another.

First, you check that dhcp installed the default route, and do you tests using IP addresses only.

Then, you decide which DNS server you want to use, and configure appropriately. You will be then able to use names and browse the internet normally.

If you are not familiar about these networking things, hire a reputable professional for the job.

dpatkins Wed, 07/29/2009 - 13:22

Thank you for your time. SInce there was no way to get the DNS request from the DHCP client to go from the inside interface to the outside, we managed to get this working using a free DNS server.

Davy Ad Wed, 07/29/2009 - 15:09

Hello DWANE,

I would like to help you solve this issue but i could not really understand what you want. I think it would be nice if you can send Diagram of what ou want to do.




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