Orbi + Comcast cable modem in Bridge Mode

[pitosalas]

I have a Comcast/XFinity Cable Modem. It's only connection is an Orbi (Netgear) mesh wifi device. I am trying to set up DDNS. While I have a good high level understanding I don't have enough to sort this one out.

1. When I am configuring DDNS, does the Cable Modem (Switch) have the public network IP or  does is it the Wifi Router?

2. It seems from the ORBI/Netgear admin UI that they insist that I have only one of Netgear, No-Ip or Dyn.com. I have a namecheap name and I wanted to create a subdomain from that name to my DDNS name but I am a little stumped.

https://i.stack.imgur.com/sxKNG.png

Thanks!

[CrimpOn]

The goal is to have the Orbi be "the router", and thus have the public IP address.  The Comcast unit needs to be in "bridge" or "passthrough" mode.

Netgear is pretty clear that only those choices for DDNS will work.  If someone has found a way to make a different DDNS service work, I hope they will post instructions.

Good Luck

[pitosalas]

Thanks... that's kind of what I believed. But when I did that all kinds of odd things were happened (cause and effect not fully established though 🙂 On the other hand when it is not in bridge mode everything seems to work fine, i.e. having two routers is not breaking anything I can notice. 

I don't just want to get it to work, I'd like to understand what's going on!

[FURRYe38]

Modem Combo Units:
This would be a double NAT condition which isn't recommended. https://kb.netgear.com/30186/What-is-Double-NAT
Couple of options,
1. Configure the modem for transparent bridge mode. Then use the Orbi router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.
2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ/ExposedHost or IP Pass-Through for the IP address the Orbi router gets from the modem. Then you can use the Orbi router in Router mode.
3. Or disable all wifi radios on the modem and connect the Orbi router to the modem, configure AP mode on the Orbi router. https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point and https://www.youtube.com/watch?v=H7LOcJ8GdDo&app=desktop

[CrimpOn]

My Orbi ran as a router behind another router for two years, and I was perfectly happy.  However, I never attempted to set up things like port forwarding, VPN, etc.  Netgear ships the Orbi with router as the default and thousands of customers happily plug them into "whatever they already have" from their ISP.   If it wouldn't work at all, there would be mass hysteria.  On the other hand, those thousands of customers seldom attempt things like running their own web servers, wanting to access security cameras in a way different than the camera people intend, connecting from far away, etc.

The key to all this is Netword Address Translation (NAT).  The ISP sees only one IP address (the "public" one).  Every packet comes from that address, but from a different "port".  The NAT part of the router keeps track of which port belongs to which device on the back side.  So, when a packet comes back addressed to that port, the IP address is translated to the private IP address, the port number is translated, and the packet delivered.  For very simple things, "double NAT" causes no problems, but for anything complicated it causes horrible problems.

There are some great articles on the web about "double NAT and why it is a problem".

[pitosalas]

Thanks for the excellent clarifications, both of you! Of the three options offered by Furrye38 is there a preferred one?

[CrimpOn]

Since the goal is DDNS, putting the Comcast in bridge mode is the way to go.

[FURRYe38]

NAT is not the problem. The problem is that users don't understand how NAT works or the fundamentals of having two routers on the same line. Some users presume that this configuration works. And for the most part it does. However, like you said, you didn't attempt any port fowards or probably didn't have any kind of networking that depended upon having a single NAT configuration. Like gaming for instance depends on a single NAT condition. 

Having two routers on line is OK. Simple solutions like using the 1st routers DMZ for the 2nd router helps with NAT conditions. 

Overall having two routers online at the same time is not recommended or perferred for home use. If your an advanced user, then thats your choice. Having two routers online could be comproblematic and troubleshooting could be a hassle. So this is another reason why we have users bridge there modem, or use the DMZ for Orbi connected systems. In some cases if the front router isn't configurable, then AP mode would be needed on the Orbi. 

Over all, general knowledge of users modems seems to be if it works then Orbi should work. It's take a bit more information gathering to help user get to a perferred configuration with there ISP modems and Orbi. This only applies to users with modem/router combo units.