CGNAT vs Double NAT: How to Tell the Difference and Fix Both

February 15, 2026

If your NAT type stays strict no matter what router settings you change, you may be dealing with CGNAT, double NAT, or both.

They sound similar, but they are different problems with different fixes.

This guide gives you a practical, no-fluff way to identify which one you have and what to do next.

Quick Definitions

What is Double NAT?

Double NAT happens when traffic passes through two routers that both perform NAT.

Example:

  • ISP modem/router (NAT #1)
  • Your own gaming router (NAT #2)

Common symptoms:

  • Moderate/Strict NAT in games
  • Port forwarding seems unreliable
  • UPnP works inconsistently

What is CGNAT?

CGNAT (Carrier-Grade NAT) means your ISP places your connection behind shared public IP infrastructure.

You may have perfect home router settings and still not get an open inbound path.

Common symptoms:

  • Port forwarding does not work even when configured correctly
  • Remote access/server hosting fails
  • You cannot obtain a true public IPv4 on your WAN interface

If you need NAT basics first: What Is NAT Type?

CGNAT vs Double NAT: Fast Comparison

| Item | Double NAT | CGNAT | |---|---|---| | Where it occurs | Inside your home network | At ISP network layer | | Can router settings alone fix it? | Often yes | Usually no | | Port forwarding success | Sometimes | Usually fails for inbound | | Typical fix | Bridge mode / single NAT gateway | Request public IP / static IP / business plan / IPv6 strategy |

How to Tell Which One You Have

Step 1) Check your router WAN IP

Open your router admin panel and find the WAN/Internet IP.

  • If WAN IP is private range (10.x.x.x, 172.16–31.x.x, 192.168.x.x, or 100.64–100.127.x.x), you likely have upstream NAT.
  • If your WAN IP differs from the public IP shown by web tools, you may be behind ISP-level NAT (often CGNAT).

Step 2) Inspect your physical topology

If you have both ISP modem/router and your own router in router mode, double NAT is likely.

Typical double NAT setup:

Internet -> ISP router (NAT) -> Your router (NAT) -> devices

Step 3) Test port forwarding with a controlled rule

Set a known-forwarded port to one device and test from outside.

  • If you cleaned home topology and forwarding still fails, CGNAT is likely the blocker.

How to Fix Double NAT

Option A: Put ISP device in bridge mode (best)

  • ISP device becomes modem only
  • Your router does all NAT/firewall/QoS
  • Cleaner and most predictable for gaming

Option B: Put your router in access point mode

  • ISP router remains primary NAT
  • You lose some advanced router features
  • Simple, stable fallback for non-advanced users

Option C: DMZ your router on ISP gateway

  • Intermediate workaround when bridge mode is unavailable
  • Better than unmanaged double NAT, but still not as clean as true bridge mode

Related: DMZ vs Port Forwarding vs UPnP

And full troubleshooting flow: Double NAT Issues and Solutions

How to Fix CGNAT

CGNAT is controlled by your ISP. Router tweaks at home usually cannot fully solve inbound access.

What works

  1. Ask ISP for a public IPv4 (dynamic or static)
  2. Upgrade to plan tier that includes public IP
  3. Use IPv6-native services where possible
  4. Use a tunnel/relay/VPN architecture for remote access and self-hosted services

What usually does not work

  • Repeatedly changing UPnP and port forwarding rules at home
  • Factory-resetting router without changing ISP provisioning

Gaming Impact: Which Problem Hurts More?

Both can cause strict/moderate NAT behavior, but:

  • Double NAT is often fixable by home topology changes
  • CGNAT often requires ISP cooperation or architecture workarounds

For matchmaking-heavy titles and party chat, eliminating double NAT first gives quick wins. For hosting sessions/servers from home, CGNAT is often the hard blocker.

See also: Best NAT Type Settings by Game

Recommended Troubleshooting Order

  1. Map your network physically (what devices route?)
  2. Remove double NAT first (bridge mode or AP mode)
  3. Retest NAT and port behavior
  4. If still blocked, confirm CGNAT with ISP
  5. Request public IP or implement relay strategy

This order avoids wasting hours on router tweaks that cannot bypass ISP-level NAT.

FAQ

Can I have both CGNAT and double NAT at the same time?

Yes. This is common. Your home can have two NAT layers, and your ISP can add another upstream layer.

Does IPv6 eliminate NAT problems completely?

Not completely, but IPv6 can reduce reliance on NAT for many services. Real-world behavior still depends on game/app support and firewall policy.

Is UPnP enough to fix these issues?

UPnP helps local port mapping. It does not remove ISP-level CGNAT.

Why does one game work and another fail on the same network?

Different games use different network architectures (dedicated servers, P2P, relay systems, party/voice channels), so NAT sensitivity varies.

Final Takeaway

If your NAT issues feel random, split the diagnosis into two buckets:

  • Home topology issue (double NAT): usually fixable by network design
  • ISP architecture issue (CGNAT): usually fixable only with ISP/public IP changes

Solve in that order and your results improve much faster.

If you are currently troubleshooting strict NAT, start with our full NAT Type Troubleshooting Guide.