The target here is a machine that's turned off (we want to wake it up) and many switches clear out entries from their ARP cache after a period of inactivity, commonly four hours. This would seem to be foolproof, but remember that when IP packets are delivered, the Address Resolution Protocol maps the IP address to the MAC of the client and switches maintain this ARP cache of mappings to know targets that it can deliver to. With unicast, a magic packet (a special-format packet that has a sequence of bytes-the client's MAC address-repeated 16 times, like someone waking you up by saying your name repeatedly) is sent directly to the IP address of the client. SCCM 2007 offers two WOL configurations, which are configured on a per SCCM site basis-unicast and subnet-directed broadcast.
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