Hi! This page dates from around ~2002 or so it is painfully out of date. The steps would still work, but the issue of an ISP blocking home routers is (hopefully) a thing of the past. I'm leaving the page up as a historical artifact at this point. -Eli

Getting a Linksys Router to work with Comcast

Here are the steps that I have had success with to get a Linksys router to talk to a Comcast cable modem. Comcast seems to be using some kind of security settings, which on the telephone they will swear do not exist, apparently for the purpose of preventing people from hooking routers up.

Their usual sleight-of-hand is as follows: they ask you to plug your computer directly into the modem. When this works, they claim that the service is operating properly and then quite happily direct you to talk to the company that manufactured your router. I know this for absolute fact, having talked to them myself. Afterward, I took the "malfunctioning" equipment and tested it on a known good network. The "broken" router always works perfectly.

My assumption is that this is to stop people from running many computers off of one connection. That is valid, I guess. Technically, it is against the EULA for the customer to do that. However: with the amount of viruses and worms and all the other baddies that are out there, I believe it important for everyone to use some sort of firewall if at all possible. It is a condom for the Internet.

These are the steps that I have to use to get the router to function again. It may vary a little bit if you have a different model of router, but the spirit remains the same:

Plug your computer directly into the cable modem.
Use "ipconfig /release" and "ipconfig /renew" from the windows command prompt to get a new IP address from the cable modem.
Unplug your computer from the cable modem.
Plug your computer into the router, leaving the router unplugged from the cable modem.
Use ipconfig /release and ipconfig /renew to get an IP address from the router.
Enter the router's configuration with your web browser.
Tell the router to Clone your Mac Address. wait a few seconds.
Plug the router into the cable modem.
In the web interface, tell the router to ask for a new IP address from the cable modem. (This is the "DHCP Renew" Button) The cable modem should think that you are the PC that just spoke with it and will let your router work again. You might have to ask it to renew a couple times.

This whole process takes a couple minutes. If you find yourself doing it a lot, I suggest purchasing a small office UPS to power the cable modem from. If the cable modem doesn't ever have a power flicker reboot it shouldn't forget about you.

The exact models of the devices involved:
- Scientific Atlanta Model DPX110 Cable Modem
- Linksys Etherfast Cable/DSL Router with 4-Port Switch Model BEFSR41

I have also had this problem with a wireless router from D-Link and a wired router from Belkin.

Updates/Refinements: (7/24/2006)
I don't have any significant proof for either of these steps, but from time to time they seem to help.

Before starting, it may help to turn the power of the cable modem and router completely off, in order to flush out any preconceptions they might have.
After step 2, when you have an ip address directly from the cable modem, it appears to occasionally help if you initiate some normal web traffic. The cable modem seems to give me less trouble if I have some normal use under my belt before I move on to step 3.

Update: (10/19/2007)
I got an email from someone who describes a problem (and solution) that sounds exactly like the sort of thing my steps above would work around:

I had a problem w/ a friend’s Comcast hook up. I read your article. I could connect to the router but not surf. The fix was simple. The idiot’s at Comcast forgot to change the type of service from “ to HN (Home network). It seems the that when you use the linksys you are considered to have a “Home Network” and it has to be changed to a different setting. When the tech figured this out (after 1 ½ hours on the phone) and we got it changed, the service was fine. The problem started when my friend switched from just Comcast high speed to their triple play. They had to switch out the regular modem for the linksys. The installer came in and got it working. It worked for a few days then it stopped working. For some unknown reason someone must have undone the changes made by the installer. I finally got it all working, including the wireless. The original installer never gave my friend the Router login & password (L/I comcast P/W 1234). What a pain in the ass this whole thing was.

... as the steps above serve to trick the comcast equipment into thinking that there is no router in the loop, I think it very likely that having that bit of information to run by the Comcast tech would have solved my original problem. If you have this sort of issue, be sure to ask the Comcast tech to verify that the type of service is set correctly on their end.