A Quick Note About Comcast Cable Modems
Posted by Andrew Pilsch on Monday, August 4th, 2008, at 12:04 pm, and tagged as broadband, cable, comcast, router.
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So I loaded Tomato onto my Linksys router this morning. After rebooting it and getting everything set up again, I noticed I couldn’t connect to the Internet. Crap.
I remembered that when my cable had been installed, I’d been “pushed through” (I think that’s Comcast’s term) by Shawna’s laptop. After the tech left, I couldn’t connect my router to the Internet, so I spoofed her MAC address with the Linksys and have not had trouble since. Logically, I assumed that Comcast somehow stores the MAC address that first connects to the Internet and you just have to use that for all eternity. Turns out I was wrong.
Loading Tomato wiped out the old, spoofed MAC address and I didn’t have Shawna’s laptop around to check the new MAC (also, I wasn’t sure I actually used Shawna’s MAC; it might have been the tech’s laptop; getting cable out in Bellefonte, that day, was a wild and weird experience), so I called Comcast. The rep said that Comcast doesn’t store MAC addresses and that I should just reset the modem and my computer with my laptop (a MacBook) connected directly to the router. Rebooted modem, rebooted computer: everything works. Connected up the router and no Internet. Then, I spoofed my computer’s MAC address and got back online.
I may be going out on a limb here, but it looks like Comcast (and maybe all cable modems (?)) grab the MAC address that’s connected to them when they turn on and will only authorize that MAC. In any case, if you ever run into such a situation as mine, try rebooting the modem.
I hope this helps someone, someday.
Image Credit: cable modem and wi-fi router by osde8info
Gerrit DeWitt said:
Hey Andrew. Gerrit here. :)
Yeah, that’s normal.
The way I see it, most of these “modems” fall into one of two categories:
A. those that can perform layer 3 functions such as routing and NAT
B. those that act as layer 2 bridges
Sounds like you have number B. :)
DSL devices are usually type A – NAT routers – because the communication to the service provider is usually done via PPPoE. It’s usually easier for customers to go to http://yeahigotdsl.com and enter a name and password than to configure PPPoE on their router or computer. These NAT routers can also be told to go into a bridge mode for another router to perform the PPPoE “dial in.”
Cable “modems” are usually type B. The big clue is if you plug your computer into the modem, you’ll get a public IP address via DHCP. So, no NAT there. The DHCP assignment is usually not coming from the “modem” – rather, it just serves as a bridge to the ISP’s network, which is where the DCHP server resides.
In either case, when operating in bridge mode, the only identifiers that the “modem” and device behind it have are the MAC addresses – the Ethernet frames are addressed this way. That much makes sense.
Now our friend OSI layer 2 has two parts: the top-part is the logical link control sublayer (LLC), and the bottom part controls MAC addressing. All of a switch’s or bridge’s real magic lies in the LLC.
A regular Ethernet switch will let you unplug and connect new devices without rebooting it. And a switch is better than a hub, because a hub is a layer 1 device that sends responses to everyone – e.g. no one-on-one communication between sender and recipient (no unicast). So “regular” switches are good for unicast, multicast, and broadcast communication at layer 2. This behavior is an unacknowledged, connectionless mode, which sends data efficiently to one or more recipients, but doesn’t verify that the recipients would get the data. Verification could be left up to an upper-layer service instead. This behavior is defined in IEEE 802.2, which is LLC.
Enough about a regular switch.
IEEE 802.2 also defines two other LLC control methods, one of which is an acknowledged, connectionless mode which only supports one-on-one communication. For the service to be acknowledged, each of the two devices in the “one-on-one chain” need to know about each other. Since we’re still in layer 2, the only way to know one another is via MAC address.
So that’s why the router “caches” the MAC.
For the curious, LLC can also operate in a connection-oriented mode, where packet order is maintained.
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