Networking – Configure DD-WRT router behind a Cisco cable modem

As heavyd said, the address for the modem needs to be on a different subnet than the one controlled by your router.

Just to clarify, the subnet mask shows which part of the IP address is the subnet. A subnet mask of 255.255.255.0 means the first three numbers in the IP address are the subnet.

So in your setup, both the router and the modem have the subnet 10.0.0.x. When you enter the address 10.0.0.1, the router sees that address is on the subnet it controls and never sends packets for that address out on its WAN port.

So, as heavyd said, the solution is to change the router's IP address to any private IP address that doesn't begin with 10.0.0.

For example, you could change your router's IP address to 10.3.3.3 and leave its subnet mask at 255.255.255.0. In that case the router's subnet would be 10.3.3.x. Then when you enter the address 10.0.0.1, the router will see that address isn't on the subnet it controls and will send packets for that address onward to its WAN port.

I'm thinking that things were good until your ISP changed your IP address, gateway, or DNS servers recently. Check your computer's host file. Could you have some ancient settings in there that are blocking things? Also, could you have some obsolete firewall settings?

Also, make sure that your ISP is giving you valid DNS server addresses, and that you are using them. The ones in your listing don't work for me, but I think it's just because I'm not on comcast.

iMacCoreDuo:~ jamiecox$ nslookup www.apple.com 68.87.76.178
Server: 68.87.76.178 Address: 68.87.76.178#53

** server can't find www.apple.com.cfl.rr.com: REFUSED

But, if they don't work from Comcast, maybe they are obsolete.

So, do
nslookup www.google.com

and see what your get. Then try
nslookup www.google.com 192.168.1.1

this will be asking your router instead of the external DNS servers.

Added this:

It seems obvious, but check that things are cabled up right. Is the cable modem connected to the WAN port of the router? It should be. Is the computer connected to a normal LAN port -- not designated as a monitor port or something?

I suggest that if you don't need to be doing MAC Address cloning, that you turn that off. It could be causing a weird problem.

If I was your ISP, and I wanted to create this situation, (say I didn't want you to use a router, or I wanted to force you to buy my router), I would make sure that all inbound IP traffic coming out of your cable modem had TTL (time to live) of 1. Any such traffic would be (correctly) dropped by the first router it hit. Your outbound traffic would still work, but you just couldn't get anything back. This would be an underhanded trick, and I've never heard of anyone doing it, but it would exactly explain your symptoms.

Check to see if the PC and the router can ping their default gateways. This should be the easiest thing to get working. Then see if the PC can ping the external gateway (the one the router sees).