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 192.168.1.1 255.255.255.0 # 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 full-duplex ! ip nat inside source list 1 interface Ethernet0/0 overload ip classless no ip http server ip pim bidir-enable ! access-list 1 permit 192.168.1.0 0.0.0.255
Last line defines the access list. It means that all hosts in 192.168.1.0/24 range will be alowed to use the NAT. The subnet mask needs to go in reverse, so 255.0.0.0 is 0.0.0.255.