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Cisco: Cisco router as NAT device using COMCAST cable modem

by Kliment Andreev

I bought an used Cisco 2611 router from eBay to use it on my internal network. While I was playing with it, I decided to try and see if I can get it working as a NAT device instead of my current FreeBSD box. You will still need the modem that your ISP provides it to you. I doubt that you can use a coax Cisco module and replace the ISP’s modem. Comcast uses modem’s MAC address to track your connection, usage etc…

So I have two NICs in the router and this is how they are confugured.

interface Ethernet0/0
description OUTSIDE
ip address dhcp   #The outside interface gets its IP address from the modem since I don;t have a static IP
no ip unreachables
no ip proxy-arp
ip nat outside  # Define the NAT
half-duplex  # For some reason, if I used auto or full-duplex, the connection was painfully slow
no cdp enable  # You don’t need Cisco CDP protocol running on the outside interface
interface Ethernet0/1
description INSIDE
ip address # My internal IP, this is going to be your gateway IP
no ip proxy-arp
ip nat inside # Define the NAT on the inside interface
ip nat inside source list 1 interface Ethernet0/0 overload
ip classless
no ip http server
ip pim bidir-enable
access-list 1 permit

Last line defines the access list. It means that all hosts in range will be alowed to use the NAT. The subnet mask needs to go in reverse, so is

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