WOL Outage Fix and Network Analysis Report (Including Test Results)

2대의 서버간 패킷을 전달하는 트럭 이미지

1. Overview

  • Sender: Raspberry Pi 5 (192.168.0.xxx)
  • Receiver: RTX 2060 PC (MAC: AB:CD:EF:GH:IJ:KL, IP: 192.168.0.xxx)
  • Network change:
  • Normal: Raspi and the 2060 PC were connected to the same router (Bridge B) in the same physical segment.
  • Problem: The 2060 PC was moved to the upper‑level router (A) directly, separating it from Bridge B.
  • Symptom: The usual WOL command (255.255.255.255) no longer powers on the PC.

2. Symptoms

Failing case: wakeonlan [MAC] (Limited Broadcast)

  • Destination: 255.255.255.255:9
  • Result: No response from the 2060 PC.

Successful case: wakeonlan -i 192.168.0.255 [MAC] (Directed Broadcast)

  • Destination: 192.168.0.255:9
  • Result: The 2060 PC powers on.

3. Experiments and Verification

✅ Experiment 1: Verify packet transmission from the sender (Raspi)

  • Goal: Rule out OS or software‑level transmission errors.
  • Method: Monitor the interface with tcpdump.
# Raspi terminal
$ sudo tcpdump -ni eth0 udp port 9
19:25:35.861285 IP 192.168.0.xxx.52072 > 255.255.255.255.9: UDP, length 102
  • Conclusion: The Raspi generates and sends the packet correctly.

✅ Experiment 2: Check if the packet reaches the receiver (2060 PC)

  • Goal: Determine whether the TP‑Link bridge blocks the broadcast.
  • Method: Run tcpdump on the 2060 PC while the Raspi sends the default command.
# 2060 PC terminal (listening)
$ sudo tcpdump -ni enp5s0 udp port 9
listening on enp5s0...
# (Result: No packets captured)
  • Conclusion: Packets addressed to 255.255.255.255 do not pass the Bridge B segment.

4. Root Cause

image The issue stems from the broadcast scope and the forwarding policy of the bridge device.

Type Limited Broadcast (255.255.255.255) Directed Broadcast (192.168.0.255)
Meaning "Only within this physical link" "The entire network that uses this IP range"
Bridge behavior Treated as local traffic, not forwarded upstream. Recognized as valid network traffic and forwarded.
WOL outcome Failed – blocked at the network boundary. Successful – reaches the 2060 PC via router A.

5. Resolution

  • Fix: Specify the subnet broadcast address with -i 192.168.0.255.
  • Convenience: Add an alias to .bashrc.
alias wake2060='wakeonlan -i 192.168.0.255 ab:cd:ef:gh:ij:kl'

💡 Final Insight

255.255.255.255 is a Local/Limited Broadcast; it cannot cross physical network boundaries. In segmented environments (router‑level bridges, etc.), using a targeted subnet broadcast (192…255) provides reliable delivery.