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I am attempting to send WOL packets to a series of machines, which works when the machine sending the packet is connected to the destination by one or more switches, but if the packet needs to cross a router it is eaten. The people responsible for configuring and maintaining the routers/switches are telling me that the routers are eating the WOL packets because they are sent as broadcasts.

So I would like to know if anyone knows how I could send the WOL frame to a machine on the other side of a router without it being wrapped in a broadcast IP packet.

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Wake-On-LAN uses a "magic packet" sent at Layer 2 of the OSI model. Routers need information that is contained at Layer 3 of the OSI model in order to route packets. Switches on the other had are Layer 2 devices, which is why it works when you are just going through a switch. In short, you need to have IP information in order to cross a router, WOL only has MAC address information, so it is not possible to cross the router.

Further, the computer that you are wanting to wake is not turned on, and therefore does not have an IP address, so it would be impossible to send a packet and have it cross the router and reach your computer short of configuring your router to statically route the packet to your NIC's MAC (see Wake-on-Internet for additional options).

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  • Thank you, your first paragraph confirms what I thought was going on. As for the second part, I wonder if there are any NICs/Motherboards which are capable of maintaining an IP when "powered off"... – patrick-mckinnon Feb 4 '10 at 16:54
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Even though the previous answer makes sense, it is not true that a magic-packet can't cross a router. Try Wake On LAN 2 to power up a machine in local network. I was be able to run it from a PC, connected via WLAN, to power up another machine plugged in a switch which was plugged in a router. Not sure if static ip-address in router did the trick. Maybe the software is sending on 192.168.0.255 (a networks broadcast-address) to reach all connected adapters.

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Usually routers block packets sent to broadcast IP addresses like 192.168.0.255 (VPN connections as well).

But magic packets can cross the router if it allows static entries to be added to its ARP table:

  • You can create one more broadcast address for your router. For example, add this ARP entry: 192.168.0.254 FF:FF:FF:FF:FF:FF.
  • Another way is to assign MAC address to your destination IP address: 192.168.0.X XX:XX:XX:XX:XX:XX.

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