MagicPacket is a format specified by AMD for waking up remote computers via WakeOnLan.
It simply consists of 6 repititions of 0xFF, followed by 16 repititions of the IEEE address of the machine you are trying to wake up. (On an Ethernet, this refers to the MAC address.) This pattern be included in the payload of any type of packet as long as it will be seen by the destination machine (so a unicast packet to a third machine won't help if you are on a switched Ethernet).
See also:
AMD's notes on MagicPacketetherwake, a tool to send MagicPacketsOne page links to MagicPacket:
