Help with Motorola SB5120

Let's alter that quote just a little to make it clearer:

"There are no changes that can be made BY END USERS in the configuration menus of the SBG5120 Cable Modem other then resetting the modem."

Changes to the modem configuration are made by the owners of the network that it is placed on: That is, the cable company. A configuration file is downloaded to the cable modem whenever it connects after being reset. You, the end user, do not get to configure any component on the cable company's network, including the cable modem that you may have bought to place on that network.

The ISP controls the modem. Add a router between the modem and the PC and do your config there. Netgear/Linksys routers are cheap (at least here in the US - $30-40 on sale after rebate) and they give you added security to protect your PC. Get a wireless G router if you need wireless - else just get a wired one for maybe $10 less. Either should have a 4-port switch to connect your wired computers.

If those are the only two alternatives you'll accept, I believe you'll need to return it.

Glad to help.

If those are the only alternatives the OP will accept, then the OP needs to abandon broadband internet. See Warren's post in this thread.

Right. When the modem is reset, my ISP uploads a config to the tftp server running on the cable modem. I agree that my ISP has the right to control equipment connected to it's network, including blocking certain netbios garbage.

But does my ISP have the right to configure things on my LAN ? What if someone in the same office here decides to connect to the web interface on 192.168.100.1 and keep resetting the damn thing ? Don't I have the right to configure a password on MY side of the network ? And don't I have the right to change the LAN IP from the default of 192.168.100.1 to suit the IP numbering on my LAN as I see fit? This would have nothing at all to do with my ISP, and they shouldn't even care. If I can monitor the cable modem to graph the traffic statitistics using SNMP, shouldn't I be able to set the read community string to something other than the default of "public" ?

No. And they don't.

In the same office? This isn't a residential account?

If they work in the same office as you, and are on the same LAN that you're on, then talk to your network admin or your boss about what they're doing.

If I misunderstand what you mean by "the same office", and you meant someone else on the same node as you on the cable company's HFC network, then it's not going to happen that way. Someone else looking for 192.168.100.1 won't see your modem's interface. They'll see their modem's interface.

You can do whatever you want on your side of the network, but I haven't seen any cablemodems that allow you to change the IP address of their web interface. But if you place a router between your LAN and the cablenetwork, that's a moot point.

No. The cablemodem is not on your network. It is on the cable company's network. If you haven't placed a router between it and your computers, then you've placed all your computers on their network. A router would delineate where your network ends, and their network begins. Everything on one side of the router would be on your network. Everything on the other side of it would be on the cable company's network. You only get to do things on the cable company's network that they decide to allow you to do, and controlling the modem's configuration isn't one of them.

You get to stay on your side, and do whatever you want to your LAN. They get to stay on their side and do whatever they want to their WAN. The modem is part of their WAN.

I used to have a D-Link that would.

According to Motorola's own pdf datasheet, the SB5120 "Supports up to 32 users (one via USB and 31 via Ethernet or 32 users on Ethernet)"

Also, the web interface shows a screen under the "Configuration" link with the following info:

" DHCP Server Enabled The SURFboard cable modem can be used as a gateway to the Internet by a maximum of 32 users on a Local Area Network (LAN). When the Cable Modem is disconnected from the Internet, users on the LAN can be dynamically assigned IP Addresses by the Cable Modem DHCP Server. These addresses are assigned from an address pool which begins with 192.168.100.11 and ends with 192.168.100.42. Statically assigned IP addresses for other devices on the LAN should be chosen from outside of this range"

Well, 42 minus 11 equals the 31 IP addresses they're talking about in the pdf datasheet. "can be used as a gateway to the Internet" "gateway" is a router, specifically a NAT router. So what I'm trying to do is whatever I want on the LAN side. I get to stay on my side, and they get to do whatever they want to their WAN.

By the "same office", I was actually simplifying and stretching the truth a little bit. It _is_ a residential account, but will soon be part of a VPN. My whole point is that I paid for this sucker, and I do not have administrative control over the part of it that is connected to my side of the network. Whether I actually set a password on the web interface, or change the SNMP read or write strings is beside the point. I _should_ have these abilities whether I decide to use them or not.

Most cable modems support more than one user, but that requires more than one IP address from your cable provider. Cable modems are usually provisioned for only 1 IP address. The provider will probably charge additional fees to give you more IP addresses. Most people put a router (they're pretty cheap) between their LAN and the cable modem, giving them complete control without paying for extra IP addresses from the provider.

That you paid for the modem is irrelevant. You put it on their network. They get to configure things on their network.

You want to configure your cablemodem? Take it off the cable company's network. Do whatever you want to it. But once you put it back on their network, you are agreeing that they will be allowed to reconfigure it for their network.

You get to configure things on your network. But your network is on the opposite side of the router from where the cablemodem is. The cablemodem is not on your network. It's on their network.

This post was helpful, thanks. It explains why when I connect up two workstations through a switch or hub to the cable modem, I receive two dhcp assigned publicly routable IPs from my ISP. The grade of service I'm paying for allows me a max of 3.

But I am still confused about the statement I posted previously which shows up in the cable modem's web interface, stating that up to 32 dhcp addresses can be assigned, within the IP block of 192.168.100.*

I have verified that this is true. When I disconnect the cable, or kick the modem into standby, I can fire up 3 or 4 boxes and each one gets a dhcp assigned address in the 192.168.100.* range. But what use are these IPs, if they cannot be routed out and NAT translated once the cable is reconnected to the WAN ?

Lets say I hook up a router with between my LAN and the modem. The dhcp server on the cable modem seems pointless if the only way to get it working on the lan is to disconnect the WAN side temporarily. I could configure the router to NAT the assigned addresses out, and then connect up the cable again. What a mess. Better to run a dhcp server on the router and use it to NAT everything out.

I'm sure that somewhere buried in encrypted archives somewhere at Motorola, there is complete documentation on how to tweak every feature of the SB5120. But lets face it, they don't want the average end user messing with it. Unfortunate for those of us who wish to learn and experiment. Not everybody wants to hack, do damage, or violate the terms of acceptable use just for the thrill of it.

Thanks to everybody for your input, and best wishes :) I'm out of here for awhile......

Their use is two-fold: One is that when the HFC network is down, the devices on the user side of the cablemodem still have connectivity with each other. The second reason is that it was no big deal to make this capability available for up to 32 devices than it was to make it available to a more reasonable three or four devices, but 32 looks so much better to the marketing department.

The cablemodem, when connected to the HFC network, is supposed to act as a bridge. When it's disconnected from the HFC network, it's nothing but a DHCP server, which is something it's not when it's connected to the HFC network unless the admins of the HFC network want to run their network that way. (Most don't.) The cablemodem is not a router. It's not even a switch or hub. That's not how it was designed to work.

Well, let's put it this way: If you're using the cablemodem without connecting it to an HFC network, there's nothing to configure. If you're using the cablemodem connected to an HFC network, it's no more your business to configure it than it is for you to be configuring the CMTS. It has nothing to do with whether you are average, advanced, or totally unclear on the concept of how things work.

Your desire to learn does not trump the cable company's need to protect their network. If you did manager to figure out how to hack into the modem, there are absolutely no changes that you can make that wouldn't be a violation of your terms of service. The fact that you think it's virtuous to hack into a network that you have no business hacking into indicates that you just don't understand. I'd like to learn what my neighbors are doing behind their closed blinds. Is it okay that I want to peep in their window? It's not that I want to do any damage. I just want to learn. Is that wrong? You bet it is! Academic intent does not trump privacy or property rights. Hacking in to learn is just as wrong as hacking in to do damage. Your intent is irrelevant. Only your actions matter, and your actions most definitely would be a violation of the terms of service.

Your curiosity would be better aimed at learning how HFC networks operate than learning how you can misconfigure a cablemodem.

Well said. I wish the Bush administration would concur.