[nzlug] Telstra/linux

[Phillip Hutchings]

On 7/5/07, Mathew Carley <nzlug at mathew-carley.com> wrote:
> As I was trying to explain, I *think* that these modems are simply
> bridges

Not even bridges, they're media convertors. All they do is convert
ethernet over cable to ethernet over 10-BaseT, a bridge would mess
with IP level traffic. There are some added smarts for SNMP reporting
to the management systems and traffic shaping, but they're a very
basic device in network terms.

> I would think that if the modem is going to be set by anything, it would
> be by TFTP based on your node id on the cable network,

Mostly, and MAC address, there aren't node IDs on the c able network.
There are some configuration options, mostly the frequency to use,
since that can be detected incorrectly by the modem in some cases and
make startup really slow.

> itself, as it seems that the MAC addresses stored within the cable modem
> are somehow important (MAC based routing of some sort?)

MAC filtering at the head end, to make sure you can't just use a
random cable modem and someone else's IP address. I believe the IP
address you use is tied to the MAC of the modem in some way, but you
can certainly send non-IP4 packets over the cable network. When I was
on cable I had much fun with BGP4+ and IPv6 between a group of us.
Saved on traffic in the old days, but I believe the new system would
put an end to that.

> Don't ask me how I am still able to access my modem via 192.168.100.1
> when my ethernet cards IP address is not on that range... everything
> from networking 101 says that it doesn't make sense and I shouldn't be
> able to get to that address, yet I can.

Unless your internal network is 192.168.100.0/24 requests to that
address will be treated like any other request to the big bad
internet. It's just special because the cable modem itself will
intercept the packets and reply to them rather than simply forwarding
to the cable network.

-- 
Phillip Hutchings
http://www.sitharus.com/