Two Routers in One Home: When Double NAT Matters

The new Wi-Fi system may be another router

Double NAT between a mesh router and an ISP gateway can create unexpected network boundaries. Identify each device’s role and the affected function before changing modes, then verify local services and provider-dependent features as well as internet access.

A Reddit homeowner planning cameras had an ISP router and a second TP-Link router, and asked whether switching the second unit to access-point mode would help. The concern was where cameras and phones should connect. That is a topology question, not evidence that double NAT always slows internet access.

This is not automatically the cause of every slow connection. Ordinary browsing may work while gaming, inbound services, device discovery, or administration becomes confusing. Diagnose the actual failed function rather than treating the warning as a universal emergency.

Source: Would double NAT be an issue?

Confirm the upstream path, not just one address

In the authorized router interface, compare the mesh WAN address and upstream gateway with the ISP gateway’s LAN network. If the mesh WAN belongs to that LAN and both boxes have routing/NAT enabled, the home has two translation layers. Record their modes and cable connections. A private address can also come from an upstream building or provider network.

RFC 6598 reserves 100.64.0.0/10, from 100.64.0.0 through 100.127.255.255, for shared address space commonly used with carrier-grade NAT. Seeing that range does not prove a second home router. Ask the ISP about its arrangement; bridging your home equipment does not remove translation performed by the provider. Do not post administrative screenshots containing access credentials.

Source: RFC 6598: Shared Address Space for carrier-grade NAT · Google Home: Fix Double NAT when two routers run at the same time

Choose among three supported arrangements

First, the ISP gateway may support bridge mode or an ISP-specific IP passthrough configuration so the mesh performs the household routing. Those terms are not interchangeable on every product. Confirm what remains active and whether provider phone, television or authentication depends on the gateway.

Second, keep the ISP gateway as the router and use a supported mesh bridge/access-point mode. eero documents that its Wi-Fi continues in bridge mode while routing services move upstream, with some advanced features unavailable. Google Wifi, Nest Wifi and Nest Wifi Pro have a different limit: Google supports bridge mode only for a single Wifi device, not a network with multiple mesh points. It also disables Guest and Family Wi-Fi and some security protections. Google recommends bridging the ISP gateway instead. Check the exact product’s topology requirements before choosing this option. Turning off an extra Wi-Fi radio alone does not turn off its NAT.

Third, retain deliberate double NAT when the required functions work and its limitations are acceptable. Keep ordinary household devices behind the intended downstream router where practical. Inbound services, some games or cross-network discovery may need a different supported design; broad port forwarding and direct exposure of camera administration are not acceptable shortcuts.

Source: eero: What is bridge mode? · Google Home: Fix Double NAT when two routers run at the same time

Illustrative scenario: printer on the old network

Imagine phones join the new mesh network while a wired printer remains plugged into the upstream gateway. Both reach the internet, yet automatic discovery fails. The devices may sit on different local networks with a routing boundary between them.

A simpler design can place household devices behind the intended single router, with other equipment operating in a supported access-point or bridge role. The exact choice depends on ISP requirements and desired features. Preserve settings and arrange a rollback before interrupting a working connection.

Source: Google Home: Fix Double NAT when two routers run at the same time

Validate the complete household after a change

Retest internet access, local printing, smart-home discovery, remote access, and any provider phone or television service that was part of the original arrangement. Confirm that administration remains owner-controlled.

Record the final topology and where devices should connect. A successful speed test cannot establish that local discovery, security policy, and remote functions survived. If the household needs a more complex multi-network design, document those boundaries deliberately rather than leaving two routers competing by accident.

Source: Google Home: Fix Double NAT when two routers run at the same time

Put it into practice

A decision table to keep handy