System Upgrade - Cable Modem
December 11, 2002

This file is being developed. If you see this, then some old, un-related data is still present.

I was "happy" with my dial-up 56K modem internet access, but some accesses were so slow that my family decided that we should try a cable modem.

Basically, and usually on weekends, my dial-up internet connection was so slow that it was totally un-useable. For instance, signing in to a free email account required more than 30 minutes. (Yes, I know that dial-up modems are slow, but that is NOT the problem here.) During non-peak hours, access was instantaneous - maybe 20 seconds. And while some sites never loaded, other sites were still available at the same speed as before.

At any rate, my family decided that we should try a cable modem.

Getting Information

Problem 1 - no information.

Comcast was the only high speed internet option (Northern Virginia).

The sales person said that all we needed was a Cable Modem and that it would support up to 4 computers. Well, that was wrong - you can only plug in one computer. If you want to support more, you must provide some form of address translation.

Each computer on your network has its own IP address. The Cable Modem has 2 IP addresses - one for your network and the second to talk to your cable provider. The address translator converts a local network address to the internet address so that only the internet address is seen to the outside world.

This can be accomplished using one of the following.

I tried to get more information via the internet - what a joke. contains only the most worthless marketing info.
This is great - This is good - This beats DSL
I wanted Links to "product info" only apply to cable TV service - I couldn't get prices for that either.

Recently, Comcast was bought out by AT&T - so I tried their web site. Another waste of time with no useful information.

I guess persistence counts - I eventually found a list of supported cable modems (it was under FAQs).

Well, I finally got some answers at - you would think that the original comcast page would have a link to here. Instead, I had to read most of the FAQs to find it. ($55/month)

I can not determine if there is an extra charge for a fixed IP address.

Address Translation

As mentioned above, address translation is used to connect several computers to a single internet connection (cable modem).

There are basically 2 ways to provide address translation

Either way, the basic connections are (I have been told that combined Cable Modem / Router devices exist, but that they are more expensive than buying them separately, and I was not able to find one locally.) provides a very good description of address translation (NAT).

System Configuration

The relevant items are

Adding a Cable Modem

When I first connected the cable modem - it would not work.

Oh, the cable modem could see the internet ok (verified using ipconfig and the modem's lights), but Internet Explorer and ping could not.

The cable modem was plugged into the USB port, and I had the computer connected to a local network with a network card (NIC). I could see the local network fine, but not the internet.

After several hours of work, I found that when I disabled the network card (via the control panel), Internet Explorer and ping accessed the internet without problems ... but I could no longer see the computers on the network.

The solution was to enable ICS (Internet Connection Sharing).

When ICS (Internet Connection Sharing) is enabled, XP tries to set the network card to - there are no options, that is the only possible address.

Well, the existing network shared the internet through a dialup modem on another computer which already had that address. My plan was to leave the system with the modem at that address and to give the XP NIC another address. (I want to keep the dialup connection so that I could still have internet access when the cable system is not available.) You guessed it, I had to change the other computer's address.

After ICS was enable, all the computers on the local network were able to use the cable connection to access the internet and the XP system was able to see the local machines.

Cost - about 6 hours, 11 counting additional research and testing after it "worked".



Allowing for Christmas and several snow storms - 2 months is still too long.

Telnet Problem

Well, telnet worked "ok" until last Friday (06-06-03) - That's when telnet died. (There had been many times when telnet refused to work for 15 minutes to several hours, but it always fixed itself.)

About a month ago, we added a router to the network and removed the Windows XP machine from the path (we originally used it as the router, but it hangs on a regular basis and would keep the other machines from reaching the internet).

After 2 days of trouble shooting and/or waiting for the problem to fix itself, I called Comcast (the ISP). They flatly refused to talk to me - telnet is not handled by their technical support. I tried their "live chat" support, and they said that Microsoft would have to solve the problem. Specifically, they "were not licenced to support third part software".

I called Linksys (the cable modem manufacturer) - they said that as long as http, ftp, and pop3 were working, there was no way that the modem could be the problem - it just passes the packets, it does not contain a firewall or any kind of filtering capability.

I tried PortQry.exe to test port 23 (telnet)

So that's not the problem.

Princeton terminates telnet sessions in this manner for security reasons if the IP address is not properly registered.

8 hours later, it fixed itself. At 9:00 am it was broke, at 5:30 pm it worked. That's right, nobody took responsibility for the problem, it just went away on its own.

My best guess is that is was the cable company's problem.

This indicates that the problem was probably between my modem and the router shared by myself and the other computer which worked.

Of course, there is another possibility - another computer could have sent intentional disconnect messages to my system.

Trouble Shooting

Here is a great page for trouble shooting Cable Modem problems. Microsoft provides PortQry.exe (free) to test if a port on another machine is listening.

On your Windows XP system, look at

to see which ports are mapped to which services.

Author: Robert Clemenzi -
URL: http:// / user / clemenzi / technical / Upgrades / Cable_Modem.html