Wake-on-LAN is an Ethernet computer networking standard that allows a computer to be turned on or woken up by a network message. The message is usually sent by a simple program executed on another computer on the local area network. Equivalent terms include Wake-on-LAN or Wake on LAN (WOL or WoL), and Remote Wake-Up (RWU).
In case the computer being woken is communicating via Wi-Fi, the wake-up-packet can be sent via Wireless Multimedia Extensions (WMM).
 This may also be called Wake on Wireless LAN (WoWLAN).
 Wakeup over Wi-Fi networks was not possible in previous implementations of Wake-on-LAN. Wake-on-LAN is being superseded by a robust, secure, and routable out-of-band management protocol known as Desktop and mobile Architecture for System Hardware .
How it works:
Wake-on-LAN is platform-independent, so any application on any platform that sends magic packets can wake up computers running on any platform; it is not restricted only to LAN (Local area network) traffic.
The computer to be woken is first shut down (sleeping, hibernating, or soft off; i.e., ACPI state G1 or G2), with power reserved for the network card, but not disconnected from its power source. The network card listens for a specific packet containing its MAC address, called the magic packet, broadcast on the broadcast address for that particular subnet (or an entire LAN, though this requires special hardware or configuration).
The magic packet is sent on the data link or layer 2 in the OSI model and broadcast to all NICs within the network of the broadcast address; the IP-address (layer 3 in the OSI model) is not used.
When the listening computer receives this packet, the network card checks the packet for the correct information. If the magic packet is valid, the network card takes the computer out of hibernation or standby, or starts it up.
In order for Wake-on-LAN to work, parts of the network interface need to stay on. This consumes standby power, much less than normal operating power. If Wake-on-LAN is not needed, disabling it may reduce power consumption slightly while the computer is switched off but still plugged in.
Magic Packet Technology:
The Magic Packet technology is used to remotely wake up a sleeping or powered off PC on a network. This is accomplished by sending a specific packet of information, called a Magic Packet frame, to a node on the network. When a PC capable of receiving the specific frame goes to sleep, it will enable the Magic Packet mode in the LAN controller, and when the LAN controller receives a Magic Packet frame, it will alert the system to wake up. The patented Magic Packet technology is implemented entirely in the LAN controller. This architecture allows the PC to go into a very low power mode, even as far as to remove the power from the entire system, except for the LAN chip.
Wake on Internet:
The computer being woken does not know whether the wakeup signal comes from another machine on the same network or from anywhere else. If the magic packet can be made to reach a computer, it can originate anywhere (e.g., from the Internet). This can be achieved by a Virtual Private Network (VPN), which makes the remote computer appear to be a member of the Local Area Network (LAN). In the absence of a VPN, a computer connected to a router can be woken if a magic packet sent over the Internet is routed to it. This requires any firewall to be set up to allow entry of the Wake-on-LAN signal to a specified port. The port can be forwarded to the computer to be woken up; or some routers permit the packet to be broadcast to the entire LAN. However, some routers do not support this as they will not forward broadcast packets. In case the computer being woken is communicating via Wi-Fi, the wake-up-packet can be sent via Wireless Multimedia Extensions (WMM). This was not possible in previous implementations of Wake on LAN.
To make WOL work, the computer receiving the WOL request must have a motherboard and network card that supports wake on lan requests. In addition, WOL must be activated in the machine's BIOS. Most PCs today support Wake on lan.
To use Wake on Lan you will need at minimum a motherboard and a network card that support Wake on Lan. Network Cards We currently use the Intel P100+ and 3Com cards although this is by no means a recommendation.
Motherboards: There are two types of motherboard that support Wake on Lan
The two main things to remember about cabling are; 1. The majority of WoL cards require a cable connection between the motherboard and the network card. This is a three pin connection although some older cards and motherboards have two pin connections. To send the signal from the network card to the motherboard this cable has to be connected. There is a specification that would allow the card itself to pass the signal through the PCI bus but we have yet to see or test these cards or motherboards. 2. Your network card must be cabled into some form of network. This can be BNC, RJ45 or even Fibre, indeed the WoL protocol is topology and transport protocol independent so TCP/IP or IPX/SPX could be used to send the magic packet.