One of the quirks of running a home lab is that you often uncover problems you’d rarely notice on a simple home network. Recently, I ran into one of those “hidden until it hurts” issues: double NAT.
The Problem
Some of my lab services worked fine on my internal network, but remote access was a nightmare. VPN connections were unstable, and certain applications that relied on port forwarding simply refused to connect. At first, I thought it was a firewall misconfiguration or a DNS problem—but the symptoms didn’t quite fit.
The Discovery
Tracing the traffic flow helped reveal the issue. My ISP-provided modem was acting as a router and assigning private IP addresses. Meanwhile, my own router (which runs my firewall rules and VLAN setup) was also assigning private IPs. This meant I had:
Device -> My Router (NAT) -> ISP Modem (NAT) -> Internet
That’s two layers of NAT, which complicated port forwarding, broke some VPN tunnels, and caused unpredictable behavior for lab services.
The Fix
I had two options:
- Bridge Mode: Put the ISP modem into bridge mode so it passes the public IP directly to my router.
- DMZ/Pass-Through: If bridge mode isn’t available, configure the ISP router to place my router in a DMZ or use IP passthrough.
In my case, enabling bridge mode was the cleanest fix. Once my router had the public IP directly, port forwarding and VPN connections started working immediately.
Lessons Learned
- Know your network chain: ISP gear often hides extra NAT layers.
- Double NAT isn’t always obvious: Local services may work fine until you test remote access.
- Simplify when possible: One device should handle NAT, routing, and firewalling—not two.
This experience reinforced one of the biggest lessons from running a home lab: sometimes the issue isn’t exotic—it’s hiding in the fundamentals of network design.
Takeaway
If you’re running a home lab or advanced network setup, always check how many devices are performing NAT. Understanding the path your traffic takes—and who’s translating your IP addresses—can save hours of troubleshooting headaches. Sometimes, fixing a subtle double NAT is all it takes to make your lab behave predictably and reliably.