The Motorola SB5120 Cable Modem

August 2, 2013

I bought one of these for $12.90 (including shipping!) on Amazon in July of 2013. It seems to work just fine. This is a discontinued model and does only DOCSIS 2.0.

I wanted to minimize my expense of jumping from DSL to cable, figuring that I can absorb the installation charges, and upgrade the modem later.

It uses a Texas Instruments chipset and the firmware runs on VxWorks. It has a web management interface that you access at IP address 192.168.100.1. The word is that it is almost impossible to change this address, but it doesn't really matter.

Supposedly these have a user admin with default password motorola, but I have yet to be prompted for anything like this.

Surprises

The not surprising part is that even without cable connected, the web interface is available at 192.168.100.1. The interesting part is that when this thing does connect to cable it seems to give a DHCP address to the first host that asks for it, then shuts down the DHCP server and go into a "bridge" mode. This would work fine for a single computer, but if you intend to use more than one machine with it, you are going to need to buy a router (which you probably want to do anyway if you want wireless). The following link has what would seem to be a misguided discussion about how to turn off the DHCP server in the 5120: Just for completeness, note that the 5120 has a "standby" button on the top of the modem. This is a bizarre feature, but my understanding is that you can push this and the modem disconnects from the cable service giving you a warm fuzzy private feeling. This is a feature I have no intention of using. Apparently when in "standby" mode, it can and does run a DHCP server. Pretty weird.

I dug out my old Netgear MR814, which is a router with wireless and a 4 port ethernet switch. I connect the 5120 to the WAN port on this router, cycle power on the 5120, and voila, the MR614 gets an address on the cable companies LAN. I see an address like "68.226.27.211", with a 24 bit netmask. It indicates DNS servers at addresses 68.105.28.11 and 68.105.29.11. My MR814 provides me with a local network of 10.0.0.x addresses. I let it run DHCP on addresses 10 through 20, and set up the MR814 as 10.0.0.1 and my static IP machines as 10.0.0.n, where n is less than 10 -- works for me.

Wireless and the MR814

Well, as near as I can tell, the MR814 only does 802.11b. This isn't terrible (as it is 11 Mbit), but we would certainly rather have 802.11g with 54 Mbit.
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