Help! I can't escape Double NAT (Comcast/AE/Motorola SB6120)

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Help! I can't escape Double NAT (Comcast/AE/Motorola SB6120)

Hello friends,

I'm good with computers, been with Apple my whole life. But I just do not understand networking. Here's my problem...

We have Comcast high speed internet. I bought my own compatible modem, a Motorola SB6120. Connected to the modem I have an Apple Airport Extreme base station that is distributing my home WiFi network. I have no computers that run on ethernet ... everything is wireless.

I have been without working internet for several days now. The Comcast connection to my modem is good, but I just can't get my Airport to work! I keep getting a Double NAT error, can't seem to shake it. I have tried:

Disconnecting everything (modem and router), waiting 30 minutes, plugging everything back in
Restarting Airport Extreme to factory settings, setting up the WiFi network again from scratch
Trying "Bridge Mode", "DHCP only", and "DHCP and NAT"

So ... I need to know how to set up my home network so my internet will work! And I need help. I have a couple of laptops and mobile phones that regularly access my network simultaneously. So obviously my AE takes the fixed IP from Comcast, and then handles the disbursement of IPs to my devices. I just don't know how to configure everything correctly to avoid the Double NAT or whatever.

Help! Does this have anything to do with firmware? What about with OS X Mavericks? Would appreciate any help I can get! Thanks so much.

OS X Mavericks (10.9.2)

Posted on Apr 21, 2014 10:55 AM

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Answer 1

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Scott Newman

Level 2

255 points

Apr 21, 2014 12:37 PM in response to Starklassik

I've been through cable modem he** several times over the years so I understand what you're feeling.

On the surface, it should be impossible for a plain cable modem (not a wireless router) to give you a double NAT error. That said, I Googled your situation and came up with others with this problem...without an obvious solution. Given that the modem is not a router, operating your Airport unit in bridge mode won't work (as you noted).

It's probably not relevant, but I purchased an SB6120 a while back and never could get it work with my Airport Extreme (different problem).

I have two suggestions (one of which you won't like). First, open a web browser and enter http://192.168.100.1/ in the address bar. It should bring up the configuration options of the SB6120. See if there is any DCHP server setting or NAT setting that you can disable. I doubt if there is one.

Second, can you send it back and get a new SB6141? It's the easiest to install and configure of any cable modem or wireless router that I've used...and I've used several. It's also very fast. It plays very nice with my 5th gen Airport Extreme. The current Amazon price is a really good deal.

And you've already done what I would do...reset the Airport unit back to factory settings and let it try and configure automatically. Make sure the modem is connected to your Airport Extreme via the WAN port on the AE.

Google "SB6120 double NAT" (no quotes) to find people with your problem. I'm guessing you will eventually get another modem.

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Answer 2

Starklassik Author

Level 1

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Apr 21, 2014 1:34 PM in response to Scott Newman

Thank you, Scott. It's nice to know I'm not missing something horribly obvious. I've actually had this SB6120 for 2+ years (sorry that wasn't clear in the original post) and you're right ... I think I will just end up purchasing the SB6141 given its #1 status.

In the meantime I am probably going to just use an ethernet adapter...

Just to confirm ... from what the Googles tell me, "Double NAT" happens when more than one device is acting as a router? Are there devices other than modems and routers that I should be aware of that would be the source of such an error?

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Answer 3

Apr 21, 2014 5:52 PM in response to Starklassik

Starklassik,

I know a bit more than I did this afternoon. Motorola's cable modems actually do include a router that functions as a router only when the cable modem is disconnected from the Internet. Almost everyone (including me) wonders why they do this. My SB6141 has this "feature." If you Google "SB6141 DHCP server" (no quotes) you'll find lots of confused people because of this.

If an SB6120 or 6141 is connected correctly to the Internet, the internal router should not function. That's the case with my 6141. If your 6120 is connected via Comcast to the internet (all green lights or else a combination of green and blue lights) then the internal router should not be functioning and you should be able to configure your Airport device with DHCP and NAT.

Based on what I saw today during a my brief Google search, it seems that a number of 6120's are not functioning correctly when it comes to this. It seems that's the source of your error. Your Airport device is seeing the DHCP server (which shouldn't be on if your modem is connected to the Internet). You didn't say what generation your Airport Extreme is. That shouldn't be the cause of the error but who knows.

When I got my 6141 I just reset my Airport Extreme to the factory default and let it go through it's built-in configuration and I re-created by wireless network. It ended up with DHCP/NAT. That's good because there are ZERO user settings that can be changed in the 6141. The internal router function that I described above can't be turned off by the user.

One last thing. Have you connected the modem to your computer using an ethernet cable in order to verify that you do have full Internet access? If you don't have Internet access that would explain the error. If you do have Internet access then getting the 6141 is probably a good idea.

Finally, if you get a 6141 and if it's anywhere you can see it, you will likely be driven insane by the BRIGHT BLUE BLINKING light on the front that indicates data flow to and from the Internet. You're thinking "certainly it can't be that bright." It is. Wrap some white vinyl or while duct tape over the light. The light is recessed so the tape won't actually touch the light.

If you have any other questions, just post it here and I'll see it. Best of luck.

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