Cisco851 Router Working With Comcast

I got a Comcast connection the other day so now my house is dual-homed to the Internet! I have DSL with 5 static IPs (which is why I am keeping it), and now a lickity quick Comcast connection.

I did have problems getting the NATing properly working on the Cisco851 however. The first attempt had the Cisco getting the Comcast IP assigned properly w/ DHCP, and the router could ping out just fine. But, no computers behind it using NAT could get out. After much trial-and-error and dozens of google results, I got the following config that worked. I haven’t fully cleaned it up yet and fleshed out the access lists completely, but the important stuff is there 🙂

Another point to add, is that I had heard things about the MAC address getting tied to the first computer you plug in. When I was troubleshooting, I called Comcast and talked to a tech about this, and it is NOT a worry when you are plugging a router in. They claim they only use the MAC with Comcast equipment. I will say that I had first plugged a laptop in to test the line, and then I switched to the cisco without reseting the box, so my experience seemed to prove the tech’s statement.

Click here for the cisco config

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