Wake On Lan (WOL)

Wake On Lan (WOL) is an interesting concept which allows a user to power on a system from a remote machine/site.

WOL is based on the following principle:
What is WOL

"When the PC shuts down, the NIC still gets power, and keeps listening on the network for a 'magic' packet to arrive. This packet must contain a certain byte-sequence, but can be encapsulated in any kind of packet (IPX, IP, anything)"

WOL Requirements

For the WOL to work we would require the following :

ATX Motherboard
NIC (Network Interface Card) which supports WOL
We must also enable WOL in System BIOS WOL to work.
How WOL works

With WOL enabled, when you poweroff your system, the entire system goes down but the NIC still gets power and keeps listening for a "Magic" packet. As soon as it receives the packet, it know that it has to switch on the system and hence boots the system.

Magic Packet

The Magic Packet is a broadcast frame, transmitted over port 7 or 9. It can be sent over a variety of connectionless protocols (UDP, IPX) but UDP is most commonly used. In our case too we use UDP. The data that is contained in a Magic Packet is the defined constant as represented in hexadecimal: FF FF FF FF FF FF followed by sixteen repetitions of the target computer's MAC address.

The Magic Packet is actually broadcasted and not unicasted over the network. In other words the Magic Packet is a broadcast frame. The Magic Packet is broadcasted on the broadcast address for that particular subnet or the entire LAN. The listening computer receives this packet, checks it for the correct information, and then boots if the Magic Packet is valid. Since the packet contains the MAC Address of the target machine, only the machine whose MAC address matches with the one in the packet, is booted.

There are lotz of free WOL programs that are available on the net:

http://gsd.di.uminho.pt/jpo/software/wakeonlan/
http://magicpacket.free.fr/
http://www.aquilatech.com/Products/WakeOnLan/

Using these you can "wakeup" any system which is WOL compatible.