Hello @sjchavez6
Home networking is complicated. I'll try to answer this as thoroughly as I can, but this is still going to require a fair amount of information. Let's define some terms first (but fair warning: all of these definitions are "wrong" to some degree just because of the sheer complexity of this world; the definitions below as "common" and succinct, but also my own):
bridge: a device that connects two or more local area networks (LANs) to each other "transparently" (meaning it does not modify or block packets. Examples of bridges include typical Ethernet switches and modems (see below).
modem: a device compatible with your internet service provider's technology (e.g., coaxial cable, optical fiber, etc.) that adapts that technology to a standard local area network (e.g., Ethernet). From a networking perspective, this type of device is a "bridge".
router: a device that connects two or more IP networks to each other. Typically, one of these networks will be a wide area network (e.g., "the internet").
firewall: a device that blocks traffic between two or more IP networks according to configured rules. In typical consumer networking, this is a "NAT" (network address translation) firewall, meaning it allows outbound connections, but not inbound connections, and allows only inbound packets that are part of a connection that was originally initiated from "inside" the network. So-named because it hides all of the IPv4 addresses of the hosts on the inner network from the outside, behind a single IPv4 address. In home networking equipment, a "router" typically includes a "firewall" integrated together.
WiFi access point: a "bridge" that connects one or more WiFi wireless local area networks (e.g., one or more SSIDs on one or more bands like 2.4GHz, 5GHz, or 6GHz) to each other and (commonly) to one or more non-WiFi local area networks (e.g., Ethernet).
gateway: a somewhat poorly defined term that may include one or more of the above (bridge, modem, router, firewall, and WiFi access point).
double NAT: a situation where one layer of router+firewall functions is connected to the "inner" network created by another, outer layer of router+firewall functions. Double NAT often happens when an internet service provider combination device (i.e., something that includes at least a modem, router, and firewall) is installed, creating an inner network to which a third-party WiFi solution is connected – one that also includes a router+firewall feature.
Ok, so with all of those definitions out of the way, we can talk more specifically about Nest WiFi, "bridge mode" and "double NAT". In the Nest WiFi system, the "Nest WiFi Router" unit must be connected to the internet via Ethernet. By default, it will be in "router" (not "bridge") mode, creating a new inner network (wired – via the LAN Ethernet port – and wireless) with a firewall to manage access in and out. If it's connected to a simple "modem" (not a "gateway"), there will be no "double NAT" (since the modem is only acting as a bridge). This is the simplest configuration, and the most reliable.
If, however, your internet service provider has installed a more complex "gateway" that includes a modem, router+firewall, and WiFi access point all-in-one (as is increasingly common), then connecting a Nest WiFi Router to that unit will result in double NAT for the devices (wired and wireless) that are connected to the network created by the Nest WiFi Router.
There are two solutions to this:
1. Reconfigure the ISP-provided equipment to put it in "bridge" mode. This isn't always possible, but it is the simplest solution. After this change, the Nest WiFi Router is the only router+firewall in the system ("single NAT"), and it should be the only thing directly connected to the ISP's device.
2. Reconfigure the Nest WiFi Router to put it in "bridge" mode. This has significant downsides, chiefly the fact that in bridge mode, a primary Nest WiFi Router unit (the one connected to your internet service) cannot support secondary (mesh-connected) access points.
Whew. That ended up being longer than I had expected, but due to the complexity, I wanted to be as detailed as necessary.
