Folks,

I have a problem in uderstanding the way cable modem works.
At first, when it\'s not connected to the network, it has IP address of
192.168.100.1 but then when it is already connected and I am going online
to the internet, it has different IP address, let\'s say 12.34.56.68.
As far as I know, this IP address is given by server through downloading
from TFTP server.

My question is what is the difference if we ping to 192.168.100.1 and if
we ping to 12.34.56.68 ?

Any explanation would be very much appreciated.

Best Regards,
Marglione


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Cable Modem FAQ: http://www.cablemodemhelp.com
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Marglione wrote:
192.168.100.1 is the CM's ethernet address. That never changes, since
once the CM gets its IP from the DHCP server, it becomes bridged and
your ethernet traffic traverses across the cable wire. The only thing
the CM gets from the TFTP server is the config file which states the
CM's capability (BPI, up/down speed, etc). It gets this file AFTER it
gets an IP from the DHCP server. CM's really do not need an IP on
themselves, since everything is maintained via the CMTS, but CM's have
IP's for manageability.

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Isn't the 192.168.100.1 IP address usually that of the TFTP server from which
the configuration file originates?. The CM's IP address in which the CMTS
knows is its RF IP address that is on the cable side of the modem. Once the IP
address is assigned to the CM's RF interface during a registration, a DHCP
discover happens and an IP is forwarded to the CPE (the users computer). In
fact someone using the 192.168.100.1 IP figured out how load their own config
file. They did this by setting up a phony TFTP server on the CM's ethernet side
with a IP address of 192.168.100.1. Instead of getting it's config file from
the TFTP server on the RF side of the CM, it received a config file from the
dummy server. This guy was then able raise the cap on his upstream bandwidth.

TekNoid

redhat_devel wrote:

TekNoid wrote:
No, thats the Ethernet IP of the cable modem (http://192.168.100.1)
shows you your modem stats. Most DOCSIS modems have the 100.1 address
for use with their built in webserver.


yes, a 10.x.x.x address (usually)



Correct.


If the CMTS was not configured for shared key, then it may be possible.

Yes, but newer firmware is out for most DOCSIS modems and the CMTS's now
are utilizing the shared key.



--



"Windows: In a world without fences, who needs gates?"

redhat_devel wrote:

What's an Ethernet IP ? Never heard of such an animal.

$Bill wrote:
In the DOCSIS environment, the cablemodem is primarily a bridge between
two transport protocols. It has two network interfaces: The HFC
interface that the coax is connected to, and the "Ethernet" interface
that is connected to the computer (or the local LAN). Each of those
interfaces has a MAC address, and an IP address. The HFC interface
usually has a Class A private address (10.*.*.*), and the "Ethernet"
interface usually has a Class C private address (192.168.*.*).
Shorthand-wise, that makes 192.168.100.1 the "Ethernet IP." So while not
literally correct, nor the standard terminology, it's shorthand that is
usually recognized by people who work with DOCSIS modems.

--
Warren H.

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Warren wrote:

Never heard that terminology before. I would prefer Ethernet-side IP
or better still LAN-side IP. Technically, it wouldn't have to be Ethernet.

redhat_devel wrote:

Hmmm, is there a reason the cablemodem's ethernet interface has to be a
192.168.100.1 IP? Usually this is the default gateway IP for home or other network
systems. I'm not sure that all CM's follow this rule. Surely this is not a
requirement for DOCSIS modems. I looked at the DOCSIS specs and saw no reference to
this IP. Also, not all CMs have a web interface, most operators use SNMP MIBS to
access the modem for statistics.

Maybe usually in your area, but if a CMTS only assigned an IP with a subnet of
10.xxx, it would never be DOCSIS certified. Besides isn't this range also dependent
on DHCP server configuration?

I'm not sure what you mean by key, are you referring to baseline privacy? The
shared secret was always utilized (at least since I've know about CMs), the MSOs
were not changing it from the default "DOCSIS". What changed in the firmware was
the CM allowing configuration files to be accepted on the ethernet interface of the
CM. Newer firmware will only allow configuration files to be forwarded on the RF
interface. Cablelabs strongly suggested this change.

TekNoid wrote:

Vendors do this as a de-facto standard; not a DOCSIS standard.


No, and the post said it did not; just "Most". (Newer ones, mind you)

No its not; just a luxery item vendors throw into aid troubleshooting.

SNMP is not usually done by the operator anymore. CMTS's have this
function built in and poll the modems at intervals and then just dumps
them to a web page that the CMTS has built in. Makes it very easy for
the operator/MSO.


modems have the 10.x.x.x; for manageability.



no. Baseline in itself- is almost useless on DOCSIS 1.0. 1.1 and 2.0
addressed this issue.



--



"Windows: In a world without fences, who needs gates?"

We are in agreement on all points. You are correct, a cablemodem is nothing
more than a bridge. I was just nitpicking about the CM's ethernet
interface, the 192.168.100.1 "standard" IP could mislead a novice into
thinking it was hard wired. The same is true with CMTSs, 10.xxx.xxx may be a
standard here, but it does not forbid some whacky kid in asia from using
different subnetets.

TekNoid

Warren wrote:

redhat_devel wrote:

Is this the defacto standard in say, Korea?

Seems more like a security risk and a backdoor for hackers (on the CPE side only since
this IP should be unreachable from the RF (cable) side).

That's funny, Cablelabs sure spends a lot time enforcing MIB requirements and are always
proposing new objects to support equipment. Are they doing this for testability only? You
are correct though, some CMTS vendors do make it easy for operators by providing a Web
interface to configure the CMTS. Haven't heard of this done to access modems, .. but what
the hell do I know? I have DSL.

But do the MSOs use BPI or BPI+? Nice features for privacy, but I heard they were under
utilized because of complexity.

TekNoid wrote:

*Hardwired on all modems, that is ....