Dynamic PAT with DHCP assignet address support

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Nov 2nd, 2007
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Hello all

I have the following configuration but I'm not shure is this really supported on the IOS 12.3(24)I running on.

ISP/Internet--Cable Modem--Router--LAN


ISP provide dynamic assignet ip address by DHCP to the router outside interface fa0/0. Inside is a private ip range ( on fa0/1 configured. Inside private address use dynamic PAT to reach the internet via the interface fa0/0 (outside).

With this configuration I can access the internet with the web browser but I can not ping or do any vpn access to a global internet address.

When I configure a satic address to the interface fa0/0 it works but just for one address.

here the relevant configuration of the router

interface FastEthernet0/0

description Pulic Network

ip address dhcp

ip nat outside


interface FastEthernet0/1

description Privat Network

ip address

ip nat inside

ip nat inside source route-map nat interface FastEthernet0/0 overload

*** Static NAT configuration ***

(ip nat inside source static interface fa0/0)

access-list 101 permit ip

route-map nat permit 10

match ip address 101

ip route FastEthernet0/0

Thanks for any replay

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fflamslam Sat, 11/10/2007 - 15:30
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Here's the config that I use on my 1751 router. I am running 12.4 have the exact same sceanrio as you... cable modem (DHCP IP)-router-lan. See if this works for you as well. I also have a few NAT entries that I use to get back inside as well. The IP routes that point to NULL0 are known subnets that are typically spoofed (I believe).


rfuchser Tue, 11/13/2007 - 20:56
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Hello Rick

thank you for the replay. It seems this is not supported on the IOS 12.3 version. My configuration looks simalar as yours. I attached my configuration with changed passwords and ip addresses. A ping from a inside host to the outside is only working when I configure:

ip nat inside source static xxx.xxx.xxx.xxx interface fastethernet 0/0,

but i can just configure one static nat entry. As soon I configure a secondary static configuration for a inside host the first static configuration is overwriten. IOS 12.4 is not supported on my Cisco2621 router, so the last version I can use is 12.3.



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