RBR750 Port Forwarding through another router

[CargoCult]

XFinity cable modem (Technicolor model cgm4140com) with ethernet attached RBR750 in router mode.

Should it be possible to forward a port for a device with a static ip address connected to the RBR750?

So far I've tried:

1/ Assigning a static IP on the Orbi network to the Win11 PC, defining the port, and then in the Defender firewall opening that port for inbound/outbound TCP & UDP traffic to the application

2/ Disabling uPNP and setting up a port forwarding rule on the Orbi network

The application starts and listens to the port as far as I can tell

But I'm not sure how to connect the external IP of the Xfinity modem through the Orbi's IP, to my device. Is this possible? Do I need to ask Xfinity to put their modem into bridge mode? (Got lots of devices working at the moment and trying to avoid making major changes if possible, and I recall issues with some smart devices when Orbi was in AP mode)

Any guidance gratefully received.

[CrimpOn · Accepted solution]

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@CargoCult wrote:

But I'm not sure how to connect the external IP of the Xfinity modem through the Orbi's IP, to my device. Is this possible?

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Yes, this may be possible (although tedious).  The key is having access to the Internet Service Provider (ISP) device.  Since Xfinity recommends using their 'app' to install the CGM4140COM, one might infer that you can indeed access the management interface.  Once logged into it, then you can forward the port through to the Orbi RBR750 IP.

As an experiment, I have forwarded a port (I chose port 80 for the test) through three routers:

• Router 1 forwards port 80 to the WAN IP of router 2.
• Router 2 forwards port 80 to the WAN IP of router 3
• Router 3 forwards port 80 to the LAN IP of the web server

Connect to the Public IP of Router 1 and the connection goes through three routers to the web server.

It's a bit confusing.  The device is described on this Xfinity page:

https://www.xfinity.com/support/articles/broadband-gateways-userguides 

While many of the products have distinct links to download the user guide, there is no link to a guide for this product.  The table at the bottom of the page claims there indeed is a Gateway and Network Management Tool:

There is another alternative: replace the CGM4140COM with an ordinary modem compatible with Xfinity.

This web page provides a method to discover which cable modems are compatible with Xfinity:

https://www.xfinity.com/support/devices/#unauth 

Netgear lists several modems as being compatible with Xfinity:

https://www.netgear.com/home/wifi/modems/ 

Two in their description mention Xfinity Voice.  The others mention Xfinity when looking at the actual product description.

[FURRYe38]

Not with out putting the ISP router in bridge mode or try using the DMZ on the ISP Router for the Orbi RBR. 

[CrimpOn · Accepted solution]

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@CargoCult wrote:

But I'm not sure how to connect the external IP of the Xfinity modem through the Orbi's IP, to my device. Is this possible?

---

Yes, this may be possible (although tedious).  The key is having access to the Internet Service Provider (ISP) device.  Since Xfinity recommends using their 'app' to install the CGM4140COM, one might infer that you can indeed access the management interface.  Once logged into it, then you can forward the port through to the Orbi RBR750 IP.

As an experiment, I have forwarded a port (I chose port 80 for the test) through three routers:

• Router 1 forwards port 80 to the WAN IP of router 2.
• Router 2 forwards port 80 to the WAN IP of router 3
• Router 3 forwards port 80 to the LAN IP of the web server

Connect to the Public IP of Router 1 and the connection goes through three routers to the web server.

It's a bit confusing.  The device is described on this Xfinity page:

https://www.xfinity.com/support/articles/broadband-gateways-userguides 

While many of the products have distinct links to download the user guide, there is no link to a guide for this product.  The table at the bottom of the page claims there indeed is a Gateway and Network Management Tool:

There is another alternative: replace the CGM4140COM with an ordinary modem compatible with Xfinity.

This web page provides a method to discover which cable modems are compatible with Xfinity:

https://www.xfinity.com/support/devices/#unauth 

Netgear lists several modems as being compatible with Xfinity:

https://www.netgear.com/home/wifi/modems/ 

Two in their description mention Xfinity Voice.  The others mention Xfinity when looking at the actual product description.

[CargoCult]

Thanks, I hadn't considered the DMZ option. Any advice/do I need to harden my Orbi config before I do this?

Many thanks for the insight.

[CargoCult]

Thanks for this advice - I'll have a stab at doing the double port forward and post back.

Thanks again for the insight and for trying it out!

[CrimpOn]

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@CargoCult wrote:

Thanks, I hadn't considered the DMZ option. Any advice/do I need to harden my Orbi config before I do this?

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Nothing you can do. The router is designed to be exposed to the internet. DMZ is a way to expose a device on a router LAN to the internet.