Wake-on-LAN (WoL) is an Ethernet or token ring computer networking standard that allows a computer to be turned on or awakened by a network message.

• The message is usually sent to the target computer by a program executed on a device connected to the same local area network, such as a smartphone.
• It is also possible to initiate the message from another network by using subnet directed broadcasts or a WOL gateway service.
• Equivalent terms include wake on WAN, remote wake-up, power on by LAN, power up by LAN, resume by LAN, resume on LAN and wake up on LAN.

Principle of operation

• Wake-on-LAN (“WOL”) is implemented using a specially designed packet called a magic packet, which is sent to all computers in a network, among them the computer to be awakened.
• The magic packet contains the MAC address of the destination computer, an identifying number built into each network interface card (“NIC”) or other ethernet device in a computer, that enables it to be uniquely recognized and addressed on a network.
• Powered-down or turned off computers capable of Wake-on-LAN will contain network devices able to “listen” to incoming packets in low-power mode while the system is powered down.
• If a magic packet is received that is directed to the device’s MAC address, the NIC signals the computer’s power supply or motherboard to initiate system wake-up, much in the same way as pressing the power button would do.
• The magic packet is sent on the data link layer (layer 2 in the OSI model) and when sent, is broadcast to all attached devices on a given network, using the network broadcast address; the IP-address (layer 3 in the OSI model) is not used.[ad type=”banner”]

In order for Wake-on-LAN to work, parts of the network interface need to stay on. This consumes a small amount of standby power, much less than normal operating power. Disabling wake-on-LAN when not needed, can therefore very slightly reduce power consumption on computers that are switched off but still plugged into a power socket.

Magic Packet Structure
The magic packet is a broadcast frame containing anywhere within its payload 6 bytes of all 255 (FF FF FF FF FF FF in hexadecimal), followed by sixteen repetitions of the target computer’s 48-bit MAC address, for a total of 102 bytes.
Since the magic packet is only scanned for the string above, and not actually parsed by a full protocol stack, it may be sent as any network- and transport-layer protocol, although it is typically sent as a UDP datagram to port 0, 7 or 9, or directly over Ethernet as EtherType 0x0842.

A standard magic packet has the following basic limitations:

1. Requires destination computer MAC address (also may require a SecureOn password).
2. Does not provide a delivery confirmation.
3. May not work outside of the local network.
4. Requires hardware support of Wake-On-LAN on destination computer.
5. Most 802.11 wireless interfaces do not maintain a link in low power states and cannot receive a magic packet.

The Wake-on-LAN implementation is designed to be very simple and to be quickly processed by the circuitry present on the network interface card with minimal power requirement. Because Wake-on-LAN operates below the IP protocol layer the MAC address is required and makes IP addresses and DNS names meaningless.[ad type=”banner”]

// C program to remotely Power On a PC over the
// internet using the Wake-on-LAN protocol.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <sys/types.h>
 
int main()
{
    int i;
    unsigned char toSend[102],mac[6];
    struct sockaddr_in udpClient, udpServer;
    int broadcast = 1 ;
 
    // UDP Socket creation
    int udpSocket = socket(AF_INET, SOCK_DGRAM, 0);
 
    // Manipulating the Socket
    if (setsockopt(udpSocket, SOL_SOCKET, SO_BROADCAST,
                  &broadcast, sizeof broadcast) == -1)
    {
        perror("setsockopt (SO_BROADCAST)");
        exit(EXIT_FAILURE);
    }
    udpClient.sin_family = AF_INET;
    udpClient.sin_addr.s_addr = INADDR_ANY;
    udpClient.sin_port = 0;
 
    //Binding the socket
    bind(udpSocket, (struct sockaddr*)&udpClient, sizeof(udpClient));
 
    for (i=0; i<6; i++)
        toSend[i] = 0xFF;
 
    // Let the MAC Address be ab:cd:ef:gh:ij:kl
    mac[0] = 0xab;  // 1st octet of the MAC Address
    mac[1] = 0xcd;  // 2nd octet of the MAC Address
    mac[2] = 0xef;  // 3rd octet of the MAC Address
    mac[3] = 0xgh;  // 4th octet of the MAC Address
    mac[4] = 0xij;  // 5th octet of the MAC Address
    mac[5] = 0xkl;  // 6th octet of the MAC Address
 
    for (i=1; i<=16; i++)
        memcpy(&toSend[i*6], &mac, 6*sizeof(unsigned char));
 
    udpServer.sin_family = AF_INET;
 
    // Braodcast address
    udpServer.sin_addr.s_addr = inet_addr("10.89.255.255");
    udpServer.sin_port = htons(9);
 
    sendto(udpSocket, &toSend, sizeof(unsigned char) * 102, 0,
             (struct sockaddr*)&udpServer, sizeof(udpServer));
    return 0;
}

This program will power on the switched-off PC
whose MAC Address is used in this program (the 
PC and the Host computer must be connected over
LAN).