Comcast Cable Mystery Solved

I was remotely helping my friend in the US with a comcast cable connection. Her problem was that since Sunday, she couldnt get her laptop to connect to the wireless router (linksys WRT54G). When she used her internet browser, it will ask her to run a diagnostic and only after running it will she be able to connect.

This was annoying to her and so she asked me to help her out.

The first thing I did was to dig up my notes on how her network was setup. She had a linksys cable modem that connects up to the comcast cable. Then the ethernet cable connects the cable modem to the wireless router from which my friend connects her laptop.

The wireless lan router has DHCP enabled and giving out IP addresses in the range 192.168.x.x . But when I asked my friend to do an IPCONFIG /ALL, all she got was 24.x.x.x as the IP address assigned! Hmmm, was she connecting to a rouge wireless access point? Was the “network diagnositcs” that she was running part of an unknown hacker program that had somehow VPN or tunneled to another server?

I checked the 24.x.x.x ip and sure enough it belonged to Comcast. To confirm that my friend was accessing her access point, i asked that the linksys wireless router be shutdown, and we wait to see if the wireless network connection would be broken. If it is not broken, then my friends laptop was connecting to a rouge access point -which is not good of course.

Fortunately, my friend’s laptop connectivity was broken when the wireless router was turned off. So at least we can confirm that the laptop was indeed connecting to the right access point.

But then the question was why was it getting comcast DHCP ip addresses? instead of the local wireless lan IP addresses???

It took about three hours for me to figure it out. By doing a series of traceroute and pings, we were able to determine that there was a connectivity problem between the router and the cable modem.

Once I isolated this problem, it was a matter of tracing and making sure that the cables were connected properly. And then we discovered that they were not. The ethernet cable from the cable modem was connected to the LAN port of the wirelss modem. The WAN port fo the wireless router was connected to a local device! What a switch!

So the problem was solved by simply switching the two cables. And i finally figured out why the laptop was getting COMCAST’s DCHP IP….they were placed on the same LAN port as the laptop!