I have a Cisco 1811 with a Zyxel P2812HNU-F1 VDSL modem connected to it in bridge mode. The Cisco gets the public IP address through DHCP.
I wanted to access the modem on it’s local IP address of 192.168.1.254. At first, this didn’t seem possible because all traffic out of the WAN interface is translated to the public IP address. To enable access to the modem I did the following:
First, I added a secondary IP address to the WAN interface, Fastethernet0:
interface fastethernet0 ip address 192.168.1.100 255.255.255.0 secondary
Then I added a nat pool to translate internal traffic with 192.168.1.100:
ip nat pool nat_to_modem 192.168.1.100 192.168.1.100 netmask 255.255.255.0
The global NAT command requires an access list to match traffic that is to be NATted. Here I define that all traffic to 192.168.1.0/24 is to be translated:
ip access-list extended nat_to_modem permit ip any 192.168.1.0 0.0.0.255 deny ip any any
