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Cable modem, Router, Nic, SNAFU

Last response: in Networking
October 11, 2010 2:32:13 PM

I've been trying to get cable internet into my new apartment and have spent hours on the phone between the ISP customer service and tech support. It has been a week of watching the modem's online light flashing at me. Here's the basic situation. I had a problem ages ago and added a dplinc network adapter Worked fine right away. Cable company guy showed up, checked the signal and plugged in the modem. He said it would take about 5 minutes to boot and left. Of course it didn't connect. All modem lights soid except "online" which flashes.

We've attampted a lot of trouble shooting over the last week, so ask away for pertinent info.

Here's the latest info I sent to my new isp so they can forward it to the cable company so they can ignore it. Let me know if I've included any info that presents a security risk.

Thank you.


Motorola sb5101n
Customer s/n ohzr6t6m7j7
s/n 31475800280632901011008
nfc macid 0023ed8d83e9

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:\Documents and Settings\Owner>ipconfig/all

Windows IP Configuration

Host Name . . . . . . . . . . . . : trebuilt
Primary Dns Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Unknown
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No

Ethernet adapter Local Area Connection 3:

Media State . . . . . . . . . . . : Media disconnected
Description . . . . . . . . . . . : Realtek RTL8169/8110 Family Gigabit
Ethernet NIC
Physical Address. . . . . . . . . : 00-13-D3-EE-C2-AC

Ethernet adapter Local Area Connection 2:

Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : D-Link DFE-530TX PCI Fast Ethernet A
dapter (rev.C)
Physical Address. . . . . . . . . : 00-26-5A-0C-07-4C
Dhcp Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IP Address. . . . . . . . . . . . :
Subnet Mask . . . . . . . . . . . :
Default Gateway . . . . . . . . . :
DHCP Server . . . . . . . . . . . :
Lease Obtained. . . . . . . . . . : October 11, 2010 01:59:48
Lease Expires . . . . . . . . . . : October 11, 2010 02:00:08

C:\Documents and Settings\Owner>
October 11, 2010 5:00:37 PM

I assume you’re connected directly from the PC to the modem, no router.

I'll tell you what I find out odd about that ipconfig dump. (the default gateway) is the IP address of the modem itself (simple enough to check, should bring up its UI). Some modems, like the Motorola Surfboard, can be configured as either single or multiple user. In multiple user mode, they work similar to a router. For example, they provide a DHCP server and distribute IPs in their own network (192.168.100.x).

If you look closely, it appears to me your cable modem is configured for multiple user, not single user. Notice the gateway IP is, that you have a local IP address of, etc. That’s not what you want to see. You should be seeing a public IP address (e.g.,, pubic gateway (e.g.,, public DNS server (e.g.,, etc.

To see what I mean about single vs. multiple user, check out page 40 of the following Motorola Surfboard manual:

Unfortunately, I’m not familiar w/ how you control single vs. multiple user configurations on the modem (the manual only mentions you can, but provides no details). Maybe it’s controllable only by the provider (not uncommon). All I can say is the addressing you’re receiving is wrong. By all indications, it seems to be coming from the modem and instead should be coming from the ISP.
October 12, 2010 12:56:26 PM

Interesting. Thanks for the post.

I think all that means is that the modem is not connecting to the internet.

Any other ideas?
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Best solution

October 12, 2010 9:33:32 PM

Apparently the significance of my comments hasn’t hit home. My point is, your PC shouldn’t be receiving a local IP address from the cable modem. It should be receiving a *public* IP address from the ISP!

The cable modem is an ethernet bridging technology that works at the data link layer. It binds the ethernet connection from your PC to the ethernet connection of the cable company. That’s it, nothing else, that’s all it does. If it helps, you can visualize it as a “virtual ethernet cable” that runs from your house all the way to the cable company.

At no time should the cable modem be jumping in and responding to DHPC requests, distributing IPs in its own local network, assigning itself the gateway IP, etc. You would only see that type of behavior w/a cable modem that was also capable of being a router/gateway. But as far as I can tell, the SB5101N is not a router/gateway, but just a plain ol’ cable modem.

As a plain ol’ cable modem, the SB5101N should just be passing ethernet traffic between your network and the ISP’s network. Then, when your PC makes a DHCP request, it should just flow past the cable modem and be received and responded to by the ISP, who then returns a public IP assignment, their gateway IP, etc. The modem should be completely transparent, out of the picture.

But that’s not the case here. It appears your modem is intercepting those DHCP requests and responding to them itself, as if it was a router/gateway. Just look at your ipconfig dump, it doesn’t even list DNS servers! And I understand why. The cable modem doesn’t know about any DNS servers, it only knows about itself, it’s own local network. And so that’s all it’s capable of passing back, IP information related to 192.168.100.x. All it has done is created a local network between you and itself. You’re, the cable modem is (useless).

Why?! That’s the unanswered question. That’s what you need to ask the cable company. Why are you receiving a local IP from the cable modem and not a public IP from them (the ISP)?

As I asked before (and you never indicated one way or the other), see if brings up the cable modem. None of this matters if it proves NOT to be the cable modem. But I strongly suspect it is since most cable modems use the IP address by convention.

Anyway, that’s the one anomaly I see. Heck, it could be something completely different, but I’ve been dealing w/ cable modems for years, and I’ve never seen a DHCP request that resulted in an IP assignment from the cable modem’s network (192.168.100.x). That’s just bizarre. Something is amiss, but I’m not an expert on these cable modems or how the cable company configures them (btw, they do the configuration from their side, not you). I’m thinking maybe there’s something they’ve done on their side to misconfigure it but don’t realize it.

October 19, 2010 9:00:34 PM

Finally go this sorted out with the isp. It was a dhcp issue that they kept explaining wasn't the issue.

All is well after a full week of messing around.

Thanks so much for the responses.
October 22, 2010 4:46:36 AM

Best answer selected by ttbwpg.