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Can I send a Wake On LAN packet without broadcasting?

I am able to wake my computer over internet, because my computer is connected to a router, and router is connected to cable modem. Router has it's own IP, and I can send magic packet to this IP with my computer's MAC address. I completely understand it and it works.

I am wondering if it's technically possible to wake computer connected directly to cable modem by my ISP. I would probably send then my computer's MAC address to my ISP IP, but is it possible for my ISP to send magic packet to my computer over cable modem? My ISP knows my modem's MAC and my computer's MAC (unknown devices are unable to get IP from DHCP), so maybe it's possible?

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See related: superuser.com/questions/104755/… –  heavyd Sep 14 '11 at 10:47
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marked as duplicate by RedGrittyBrick, Nifle, 8088, heavyd, random Sep 15 '11 at 2:44

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up vote 3 down vote accepted

Routers work by "rewriting" packets that they forward between networks (such as your home network and the Internet). Also, the NAT function of most routers further obfuscates traffic behind the router, making everything from an external standpoint look like it's come from your router on the surface.

As such, routers hide the MAC addresses of devices "behind" them. So this creates difficulty if the ISP really wanted to send something directly to a specific MAC address in your network.

If you connect your computer directly to the cable modem, without an intervening router, and your computer gets an IP directly from your cable company's DHCP server, then it is possible.

I believe it doesn't really matter how the "wake-on-lan" data gets to a host, it can be in an IP packet. So if you had (for example), port 5000 open on your router, and set to forward to a machine in your network with IP 192.168.0.222, theoretically your ISP could send an IP packet containing wake-on-lan data to port 5000.

A protocol called ARP is used to translate IP addresses to MAC addresses. ARP queries are broadcasted, and as such won't jump subnets (i.e. routers by default won't forward broadcast traffic). When two hosts are talking to each other for the first time, an ARP query is issued by the TCP/IP stacks of the respective hosts. Successful queries are cached for a while. I forget how long is standard.

For your router to try to forward it to 192.168.0.222, 192.168.0.222's MAC address needs to be in its ARP cache. This means your router must have "spoken" with 192.168.0.222 recently before that entry in the ARP table times out (don't know the exact duration). So, continuing with our example, if you've just shut 192.168.0.222 down, the router might still try to forward incomng data from port 5000 to it until it experiences a timeout, tries another ARP request, and fails. If you clear the ARP cache on your router when you shut a machine down this won't happen. Of course, your ISP would have to be really sleazily tracking your traffic to know you had port 5000 open. But possibly a malicious third party could do it.

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The cable modem will surely work with the wake on LAN feature. All it has to do is forward the packets to the PC. The PC will wake up if the feature is enabled.

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I'm not sure if I understand correctly, but are you talking about modem or cable modem? I am talking about cable modem. –  prostynick Sep 14 '11 at 11:21
    
Sorry! I got it now... I have edited the answer –  venomrld Sep 14 '11 at 11:32
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