Connecting through a cable modem

Isaac Wingfield isw "at" witzend.com
Tue Jan 6 16:15:01 2004


>From: "Napolean Dynamite" <elvis "at" channel1.com>

>2. And now to waste your time: Is it possible to connect to a machine
>over the Internet that is directly connected to a cable modem. The cable
>modem obtains an IP address from the cable provider and the PC has a
>private IP address (192.168.1.x). I have done this with a router that
>supports port forwarding, but I don't see how to do the equivalent with
>just a cable modem. Any ideas or pertinent links are appreciated.
>
>This is Comcast using a Motorola SB5100. Unfortunately I do not have
>access to the modem to research it more thoroughly, but from what I can
>tell, it may provide DHCP server capabilities. It gets an IP address
>from Comcast (24.60.195.x), but the PC has a DHCP assigned address of
>192.168.100.1.

Something sounds peculiar here. A DOCSIS-Compliant Cable Modem (which
Comcast uses, I believe), doesn't normally behave like that. The CM is not
the place the DHCP-assigned IP terminates; it is given to the host computer
attached to the CM. At least one host connected to the CM must have a
routable IP, which must be assigned by the cable operator out of his list
of numbers.

In my case (and I use Comcast), the host I have connected to my CM has been
assigned an IP of 24.6.xxx.xxx, which is certainly a routable IP.

I'm not familiar with the Motorola unit, but the 192.168.100.1 address
sounds very much like the private IP that is assigned to *all* their CMs so
their status can be checked by the local user (that is, if you point your
browser at 192.168.100.1, you *might* see a page showing the CM's status).

Is it possible that there is a firewall/router or something similar between
the CM and your computer, which is providing DHCP? If your PC's assigned IP
really is 192.168.100.1, that is most likely the case.

Some CMs include firewall/router/DHCP facilities (extra features, not a
part of the DOCSIS standard), and while I don't know whether the SB5100 is
among them, I doubt it.

Isaac