When cascading a secondary wireless router or homelab firewall behind an ISP-supplied Optical Network Terminal (ONT) or combo gateway, network administrators frequently run into silent routing failures or unexpected subnet shifts.
Because consumer and prosumer hardware vendors overwhelmingly default to 192.168.1.1/24 or 192.168.0.1/24, connecting the LAN port of an upstream modem to the WAN port of a downstream router triggers an immediate RFC 1918 WAN/LAN subnet collision.
Vendor Auto-Shift Logic Matrix
Rather than dropping the interface completely, modern router firmware executes an automatic DHCP pool migration during boot if it detects its default gateway subnet already exists on the WAN interface:
| Hardware Vendor | Factory Default IP | Auto-Shift Fallback Subnet | Local DNS / mDNS Host |
|---|---|---|---|
| ASUS (ASUSWRT) |
192.168.50.1 / 192.168.1.1
|
192.168.2.1/24 |
router.asus.com |
| Netgear | 192.168.1.1 |
10.0.0.1/24 |
routerlogin.net |
| TP-Link | 192.168.0.1 |
192.168.1.1/24 |
tplinkwifi.net |
| Linksys | 192.168.1.1 |
10.0.1.1/24 |
myrouter.local |
| Ubiquiti (UniFi) | 192.168.1.1 |
192.168.2.1/24 |
unifi.ui.com |
For the complete 35+ vendor lookup table, default administrative ports, and factory credential specs, see the live reference at RouterIPGuide.com.
Rapid CLI Gateway & Double NAT Verification
Instead of guessing which fallback IP a router negotiated, query the routing table directly from your terminal:
Linux (iproute2) & macOS
bash
# Extract active default gateway IPv4
ip route show default | awk '{print $3}'
# Verify if Double NAT is present (check hops 1 and 2)
traceroute -n -m 4 1.1.1.1
Windows PowerShell
PowerShell
# Query next-hop default gateway
(Get-NetRoute -DestinationPrefix '0.0.0.0/0').NextHop
# Check first 4 hops for consecutive private RFC 1918 addresses
tracert -d -h 4 1.1.1.1
If both Hop 1 and Hop 2 return private IPv4 ranges (192.168.x.x, 10.x.x.x, or 172.16.x.x – 172.31.x.x), your traffic is traversing two separate NAT tables.
Resolution Architecture
Upstream Bridge Mode: Place the ISP gateway into transparent bridge mode (disabling its NAT and DHCP server) so the downstream router receives the public WAN IP directly.
Downstream AP Mode: If the ISP gateway cannot be bridged, configure the secondary router into Access Point (AP) mode and connect the ethernet backhaul strictly via LAN-to-LAN (or designated AP WAN bridge port).
Open-Source Reference Datasets
We maintain this dataset openly for network automation scripts and diagnostic tools:
Interactive Subnet & Conflict Diagnostic Hub: RouterIPGuide Troubleshooting
GitHub Repository: routeripguide/default-router-ip-reference
Kaggle Dataset Mirror: Default Router Gateway IP Matrix
Hugging Face Dataset: routeripguide/default-router-gateway-ip-matrix
Open-Source Reference Datasets
We maintain this dataset openly for network automation scripts and diagnostic tools:
Interactive Subnet & Conflict Diagnostic Hub: RouterIPGuide Troubleshooting
GitHub Repository: routeripguide/default-router-ip-reference
Kaggle Dataset Mirror: Default Router Gateway IP Matrix
Hugging Face Dataset: routeripguide/default-router-gateway-ip-matrix
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