Deco X20: Port Forwarding Fails

Network Troubleshooting

When setting up port forwarding on Deco X20 (TP-Link), we summarize a case where the destination IP changed unexpectedly after saving, causing traffic to miss the target.
This is handled as an environment-dependent incident and not claimed as a confirmed general defect of all X20 units.

Reference Case

In the TP-Link community, a closely related known case was reported for Deco X20 + Firmware 1.6.1.
In that case (Deco X20 V1 / Firmware 1.6.1), a port forwarding destination such as 192.168.1.x was rewritten to 192.168.0.x, and changing the subnet mask to 255.255.255.0 (/24) was reported to keep the expected subnet handling.

This article’s symptom can be described as similar behavior, i.e., normalization as if treated by /24: However, the Deco’s internal implementation has not been confirmed to use a fixed /24. This remains an inference from the value observed after saving.

Input:
192.168.71.66

After saving:
192.168.68.66

Reproduction Conditions

  • Device: TP-Link Deco X20
  • Firmware: 1.6.1 Build 20250910 Rel. 43292
  • WAN: PPPoE
  • WAN IPv4: <WAN public IPv4>
  • LAN:
    • Router: 192.168.68.1
    • Subnet Mask: 255.255.252.0 (/22)
  • Target: UDP 1194 port forwarding configured to 192.168.71.66:1194

Actual Issue

  • Expected forward destination: 192.168.71.66:1194
  • Actual applied after save: 192.168.68.66:1194
  • As a result, external packets were not reaching the intended host

Countermeasures

The settings were aligned in this order:

  1. Changed DHCP pool start
192.168.71.50 - 192.168.71.250
→
192.168.68.80 - 192.168.71.250
  1. Reserved the server LAN address through Deco DHCP
DHCP reservation: 192.168.68.81
  1. Unified port forwarding destination to fixed address
UDP 1194 -> 192.168.68.81:1194

Summary

Confirming the destination after saving and aligning the DHCP range, DHCP reservation, and port forwarding destination on 192.168.68.81 resolved this port forwarding failure.

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