How to Improve P2P NAT Traversal Success

Create a reproducible baseline

1. Update both clients and verify that both devices are online.
2. Record the networks, time, and failure stage.
3. Change one variable at a time and retry.
4. Move one endpoint to a mobile hotspot as a comparison.

Reduce endpoint blocking

• Verify system time and time zone.
• Allow the client and background service through Windows Firewall.
• Temporarily pause third-party security software only for diagnosis, then restore it and add narrow rules.
• Check whether VPNs or other virtual adapters changed routing.

Note: Do not leave the firewall disabled or place the PC permanently in a router DMZ.

Simplify the router path

1. Record the main router WAN address.
2. Check whether the modem is also routing and assigning a private address.
3. Where you control the setup, keep only one NAT layer by using bridge or access-point mode.
4. Try UPnP only on a trusted network and review created mappings.

Identify CGNAT or restricted networks

• The WAN address is in 100.64.0.0/10 or another private range.
• The WAN address differs from a public-IP lookup.
• A mobile hotspot or different ISP changes the result.
• Campus, hotel, and guest networks may block UDP or client-to-client traffic.

Collect useful diagnostics

• Client and operating-system versions.
• Whether failure occurs at presence, traversal, or application access.
• WAN address class and number of routing layers.
• Logs from both endpoints for the same time window.