SB6121 modem is creating double NAT for me. Can't Port Forward.

[yazyazoo]

Hello,
I have problem where I can't get Plex or Slingbox to stream out of my home network. No portforwards work. Also DDNS services don't work. I have been googling and it seems as if the problem is a double NAT is blocking me.

I have Comcast with a Motorola SB6121 modem and Asus RT N10 router with MOCA to network the whole house.

Some people say to contact Comcast to set my modem into bridge mode but I thought Modems are already in bridge mode. It's the router/modem combo's that can be changed into different modes I think.

I am about to call Comcast but I don't really know what to ask them to fix if the Modem is in bridge mode already.

Any suggestions?

 

[sinshiva]

if you are truly in a double NAT, the second router should have a WAN IP in the private address range, like 192.168.1.xxx. some modems do actually try to NAT for your network. I had a tiny little garbage ADSL modem that despite having a single LAN port and nothing more for the LAN side of the network, defaulted to functioning as a modem/router combo, so this isn't without precedent. comcast should definitely be able to walk you through putting the device into bridge mode, should google not turn anything up.

if for whatever reason this can't be achieved, you'd need to make sure the lan network of the second router is different, like 192.168.33.1, and you'd port forward what you needed on the first router to the IP assigned to the WAN interface of the second router. then you'd have to port forward again from the second router to the intended device. also, you'd need to install a DDNS client on one of your machines.

 

[yazyazoo]

if you are truly in a double NAT, the second router should have a WAN IP in the private address range, like 192.168.1.xxx. some modems do actually try to NAT for your network. I had a tiny little garbage ADSL modem that despite having a single LAN port and nothing more for the LAN side of the network, defaulted to functioning as a modem/router combo, so this isn't without precedent. comcast should definitely be able to walk you through putting the device into bridge mode, should google not turn anything up.

if for whatever reason this can't be achieved, you'd need to make sure the lan network of the second router is different, like 192.168.33.1, and you'd port forward what you needed on the first router to the IP assigned to the WAN interface of the second router. then you'd have to port forward again from the second router to the intended device. also, you'd need to install a DDNS client on one of your machines.

Thanks for the reply. So I'm a little confused. Let's say the Motorola SB6121 is giving me IP address of 192.168.100.1 and we call this first router. My Asus is the second router. I set that up to use 192.168.1.1 so DHCP from Asus names all my devices from 192.168.1.2-192.168.1.100. Let's say the computer I am trying to reach with DDNS is 192.168.1.50 using port 3000 with DDNS setup on the Asus.

I would not know how to port forward from my Motorola Cable modem to the Asus router as the Motorola doesn't have port forward in any of it's settings. If I go to 192.168.100.1 I can access Motorola config page but don't see any settings for port forwarding.

Thanks.

 

[abailey]

Hmm. I have the SB6141 which is almost identical to your modem. Your modem should be in bridge mode automatically. The way my modem works is when it does not get a signal from my cable company, it will give me a 192.168.100 address. I can use that address to get into the configuration. As soon as I plug my cable into the device and it syncs up, it reissues my router a DHCP address that is a WAN address. Thus when it gives me a 192.168.100 address I cannot get to the internet but can get to the modem configuration. Once it gives me a WAN ip address, I can get to the internet but not into the router config (which I don't need to anyhow). I have heard, however, that in some markets that the ISP's are giving out private addresses (like the 192 address type) to customers so they can save on public IP addresses. Most people never know, but if you need to do port forwarding and such, it won't work. If that is the case with you, most people have reported that if they call the company and say they need a public IP, then the ISP will give them one.

 

[jlake]

http://forums.comcast.com/t5/Basic-...ow-to-disable-SB6121-DHCP-server/td-p/1512677

 

[yazyazoo]

http://forums.comcast.com/t5/Basic-...ow-to-disable-SB6121-DHCP-server/td-p/1512677

Thanks I saw that when I googled the problem. Only thing is I don't really know what to ask Comcast to do. I think the other guy with the same problem had that issue also.

 

[abailey]

Thanks I saw that when I googled the problem. Only thing is I don't really know what to ask Comcast to do. I think the other guy with the same problem had that issue also.

When you call tell them you need a public IP Address. You are currently getting a private IP address (192.x.x.x).

 

[yazyazoo]

Thanks guys got it fixed by calling Comcast. At first they wanted to charge me tech support since my modem is owned by me. As a last resort I asked them to just send a restart on my modem since it was free.

That seemed to fix the problem. I can not use port forwarding.

Hope this helps someone who has the same problem.

 

[Terry Kennedy]

Hope this helps someone who has the same problem.

Comcast (at least in my area) has some rather quirky issues with SB61xx units. Any unit they provide is already "registered" in their system, and will sync, load a config file, and upgrade / downgrade firmware to whatever version Comcast is currently deploying.

If you buy a new modem yourself, even if you get to the Amazon product page via Comcast's own "Buy a new modem" page, the Comcast phone support people will see the modem as "not registered". However, if you use the self-activation portal (connect a PC directly to the modem and try to access a web page), Comcast will auto-register the modem and send the config file as well as any firmware upgrade / downgrade needed to match what they deploy.

There are some other paths than can happen. If you supply a modem with really old firmware (I think this only applies to the SB6120, not the newer SB6121), that firmware is so incompatible that Comcast can't push the config file to the modem. Such modems are useless on Comcast unless you have a way of getting them flashed with newer firmware without involving Comcast.

Another route is trying to use a SB61xx intended for some other cable provider. This is often the case (no pun intended) with SB6141's in the black case. The white case ones have a generic firmware.

You should go to the modem's status page at http://192.168.100.1 and check the firmware version (on the SB6141 it is in the "Help" tab for some reason). It will be something like "SB_KOMODO-1.0.6.10-SCM00-NOSH". My SB6141 from Amazon came with 1.0.6.12, but Comcast downgraded it as part of their activation process.

A modem with some other cable provider's firmware may be in routing mode. Generic SB61xx's and SB61xx's directly from Comcast are in bridge mode. The only exception is that if the modem loses link on the cable side, it will activate its internal DHCP server, which will hand out addresses between 192.168.100.11 and 192.168.100.42. That is only to prevent attached systems failing due to not receiving a DHCP address when the cable link is down. They won't be able to reach any outside addresses until the cable link comes back up and they get new addresses from a "real" DHCP server.

One confusing thing is that the cable modem will continue to respond to the 192.168.100.1 address even when it is connected to the cable company's network and Comcast has assigned you a public address.