Disabling comcast bridge-mode on a Comcast modem

July 31, 2006 2:43 PM

How do I bypass router bridging on a Comcast cable modem?

I believe Comcast turned on router bridging on my internet connection, where it prevents me from using a router on my connection to the internet. I've checked the router, upgraded the firmware, done all of the resets, and everything else seems to be fine.

This has happened to me before where Earthlink turned on bridge-mode on their DSL modem. I was eventually able to go into the controls for the modem and turn it off but not after about 3 days of complete frustration. What I need is the URL for the modem. With the Earthlink situation, I had to guilt the lady at technical support into giving it up. I'm hoping somebody here at Mefi has been through the same thing and can just pass along those numbers.

posted by destro to technology (21 comments total)

See www.portforward.com
posted by BigBrownBear at 2:46 PM on July 31, 2006

Which modem? On the Motorola 5100 that I have, the internal IP is 192.168.100.1... but I don't remember seeing any kind of user configurable options when I was in there last...
posted by curse at 2:53 PM on July 31, 2006

What curse said... I wonder if a tracert would show you the IP address of your router?
posted by Robert Angelo at 3:02 PM on July 31, 2006

curse...thanks, that's one step closer. When it happened with Earthlink, there was a special page that wasn't linked to on the router, something like 192.168.100.1/bridge.html

maybe i'll just ahve to keep poking around and hopefully stumble on it.
posted by destro at 3:10 PM on July 31, 2006

I haven't heard of this technique; how exactly does it prevent you from using a router?

That is, are you sure you're not just failing to reset the modem when you plug in the router? Some modems will not give a new DHCP lease when the MAC changes until you reset them. Still others (though I don't have one) will record the first MAC it ever sees, and "stick" with that one. In these cases, you have to set your router to pretent to have this original MAC.
posted by odinsdream at 3:33 PM on July 31, 2006

Just call Comcast support if you suspect this. They don't officially discourage routers.
posted by caddis at 3:34 PM on July 31, 2006

I had heard all sorts of horror stories about Comcast and routers but when I finally had to call them they helped me reset the MAC address for my Linksys router without a peep.
posted by astruc at 3:43 PM on July 31, 2006

Er. pretend.. not pretent.

Anyway.
posted by odinsdream at 3:44 PM on July 31, 2006

Yes, I was able to do a hard reset and it either fixed the bridging or the MAC address; one of the two.

And bridge mode essentially tellsthe modem to skip any router between the modem and one of the computers hooked into it.
posted by destro at 3:48 PM on July 31, 2006

What odin said (and I said previously); note that most cable/dsl modems can only *be* reset by powercycling them.
posted by baylink at 3:49 PM on July 31, 2006

But, on review, I believe you're misinformed about what bridging mode does. In my experience -- and I've realy only ever seen it called that on some Netopia SDSL/IDSL routers -- it bypasses the router, supplying the public IP address from the front of the box to the ethernet jack on the back, instead of routing the packets to a *separate* network on the back, be it public, or NATted and private.

It also precludes getting into the management console on the box, since it no longer has an address; I suspect a factory reset is necessary to get it back.

In my experience, though, most cable and DSL modems (certainly the Toshiba's, Ericsson's, and Motorola's that RoadRunner Tampa Bay uses as well as the Fujitsu and Westell DSL's that Verizon uses around here) *are bridges by default*; they can neither route nor NAT.
posted by baylink at 3:53 PM on July 31, 2006

If you are having trouble with multiple computers on the inside IP 192.168.2.XXX there is one thing I've found with Comcast is that they expect the same IP address with all internet packets so you have to select the option Clone Mac address.
Strictly FYI.
posted by ptm at 5:11 PM on July 31, 2006

I agree with baylink that you may not be thinking of bridge mode in the right way.

A good example of using bridge mode is when you have a cable modem/router that converts your signal into something your network can use, but you send its signal directly to a wireless router that hooks up to your network.

In this case, you might not want your wireless router supplying IPs for your network in addition to your cable modem/router doing the same thing.

Enter bridge mode. By putting the wireless router into bridge mode you essentially make it invisible to the network. It takes the IPs supplied by your cable modem and broadcasts them wirelessly. Thus, your wired/wireless computers on the network can all be on the same IP series.

In this case, bridge mode is enabled on the wireless router. The cable modem won't know anything about it and doesn't need to.
posted by qwip at 5:35 PM on July 31, 2006

And bridge mode essentially tellsthe modem to skip any router between the modem and one of the computers hooked into it.

That isn't what bridging means. The cable modem can't "skip" a router placed between it and a computer. A router is just another type of computer, and the cable modem can't tell the difference.* See qwip's explanation of actual bridging. One easy way to tell if your cable modem is acting as a bridge, though, would be to see if a computer plugged directly into the cable modem is given a Private Network IP address. If it is, your cable modem is not acting as a bridge between you and your ISP. If instead you get a public IP address, or world-routable IP address, your cable modem is acting as a bridge, as it should.

Well, in theory, one could maintain a database of router vendors and the MAC address ranges each vendor uses, and then filter based on these MACs. But, this technique isn't effective, because most (all?) routers you can buy nowadays include a "clone MAC" option which allows the end-user to specify a new MAC visible to the cable modem.
posted by odinsdream at 6:50 PM on July 31, 2006

odins: note that it is *possible* to get a public address behind a high-speed-line-terminating-device -- if the device contains a router, as the Netopia's I mentioned do.

I have a client with about 40 branch offices served by Florida Digital Network with that sort of arrangement: their box has one public address, and my router, behind it, has another -- a routable address that I can talk to from outside.
posted by baylink at 7:30 PM on July 31, 2006

True, I'm guessing a bit at what Bridge Mode means, but it certainly was the case that I had to go to a page on the Earthlink modem that was something like 192.168.1.1/bridge.html and set bridge mode to off to get it to work.

And no, power cycling did not fix the problem in either instance. It was only a hard reset that worked.
posted by destro at 9:22 PM on July 31, 2006

If you are having trouble with multiple computers on the inside IP 192.168.2.XXX there is one thing I've found with Comcast is that they expect the same IP address with all internet packets so you have to select the option Clone Mac address.
Strictly FYI.

and strictly nonsense. mac addresses and IP addresses are two separate things.

i think you might be saying this:

"i had my computer hooked directly to the cablemodem, and when i bought a router, it wouldnt work until i told the router to use the mac address of my PC."

this is because the head end for the cablemodem (or even the cablemodem itself) has learned your mac address and likely won't allow another mac to exist, mainly because they are only selling you one IP address. if you wait long enough, the mac will age out of the modem/headend and you'll be able to use whatever mac address you like. well, within reason! theoretically all mac addresses are supposed to be unique and 2 devices on the same bridged network with the same mac could really F things up.


destro: what do you mean by "hard reset"? that term usually means the same as powercycle.
posted by joeblough at 10:22 PM on July 31, 2006

I thought powercycle meant turning it off/unplugging it, waiting 5 secs and turning it back on.

By hard reset, I meant I took a pin and pushed the little button that says "reset" that reverts everything back to the defaults.
posted by destro at 8:01 AM on August 1, 2006

i see. yeah that's what powercycle means, of course. but some people use 'hard reset' to mean the same thing. most cablemodems don't have reset buttons, so i guess that's the confusion.
posted by joeblough at 11:42 AM on August 1, 2006

Ok...

Destro? Make and model of the box the cable company gave you to terminate the line?

I'm beginning to suspect, my friends, that it's a combination modem/router. Cable/DSL modems almost *never* have reset buttons -- I have never personally seen one that did, out of literally hundreds.
posted by baylink at 2:30 PM on August 1, 2006

RCA Model DCM325. It's the model that Comcast gives out.
posted by destro at 7:38 PM on August 1, 2006