Wake on LAN
Wake on Lan uses a special package to magically wake up any remote machine that supports Wake on Lan technology (which is most machines with built in LAN cards, although you often need to enable this in the BIOS first).
You need to know the IP address and the MAC address (i.e. 00:12:13:AB:D4:93) of the machine you wish to wake up. Wake on LAN listens on ports 7 and 9 by default.