Editing Wake-on-LAN (section)

==Principle of operation==
[[Ethernet]] connections, including home and work networks, wireless data networks, and the [[Internet]] itself, are based on frames sent between computers. WoL is implemented using a specially designed frame called a [[#Magic packet|magic packet]], which is commonly 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. This is an identifying number, built into each [[network interface controller]] (NIC), that enables the NIC to be uniquely recognized and addressed on a network. In computers capable of Wake-on-LAN, the NIC(s) ''listen'' to incoming packets, even when the rest of the system is powered down. If a magic packet arrives and is addressed to the device's MAC address, the NIC signals the computer's [[Power supply unit (computer)|power supply]] or [[motherboard]] to awaken. This has the same effect as pressing the power button.

The magic packet is [[Broadcasting (networking)|broadcast]] on the [[data link layer]] to all attached devices on a given network, using the network [[broadcast address]]; the [[IP address]] (which relates to the [[internet layer]]) is not used. Because Wake-on-LAN is built upon broadcast messaging, it can generally only be used within a [[subnet]]. Wake-on-LAN can, however, operate across any network in practice, given appropriate configuration and hardware, including remote wake-up across the Internet.

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]]. To further reduce power consumption, the link speed is usually reduced to the lowest possible speed (e.g. a Gigabit Ethernet NIC maintains only a 10&nbsp;Mbit/s link). Disabling Wake-on-LAN, when not needed, can slightly reduce power consumption on computers that are switched off but still plugged into a power socket.<ref>{{cite web|url=http://www.lesswatts.org/tips/ethernet.php |work=Less Watts |title=Ethernet Tips & Tricks |url-status=dead |archive-url=https://web.archive.org/web/20071126071350/http://www.lesswatts.org/tips/ethernet.php |archive-date=November 26, 2007 }}</ref> The power drain becomes a consideration on battery-powered devices such as laptops as this can deplete the battery even when the device is completely shut down.

===Magic packet===
The ''magic packet'' is a [[Ethernet frame|frame]] that is most often sent as a [[Broadcasting (networking)|broadcast]] and that contains 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 could be sent as payload of any network- and transport-layer protocol, although it is typically sent as a [[User Datagram Protocol|UDP]] [[datagram]] to [[TCP and UDP port|port]] 0 (reserved port number),<ref>{{cite web |url=https://forums.ivanti.com/s/article/Understanding-Wake-On-LAN |title=Understanding Wake On LAN |work=LANdesk.com |access-date=28 October 2015}}</ref> 7 ([[Echo Protocol]]) or 9 ([[Discard Protocol]]),<ref>{{cite web|url=https://docs.microsoft.com/en-us/mem/configmgr/core/clients/deploy/plan/plan-wake-up-clients|title=Plan how to wake up clients in Configuration Manager|date=2019-04-23|website=[[Microsoft Docs]]|access-date=2020-10-29|quote=...&nbsp;By default, traditional wake-up packets are transmitted by using UDP port 9...}}</ref> or directly over Ethernet using [[EtherType]] 0x0842.<ref>{{cite web |url=http://wiki.wireshark.org/WakeOnLAN |title=WakeOnLAN |website=Wireshark wiki |access-date=2023-09-27}}</ref> A [[connection-oriented]] transport-layer protocol like [[Transmission Control Protocol|TCP]] is less suited for this task as it requires establishing an active connection before sending user data.

A standard magic packet has the following basic limitations:
* Requires destination computer MAC address (also may require a ''SecureOn'' password)
* Does not provide a delivery confirmation
* May not work outside of the [[local area network]]
* Requires hardware support for Wake-on-LAN in the destination computer
* 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 simple and to be quickly processed by the circuitry present on the network interface controller using minimal power. Because Wake-on-LAN operates below the IP protocol layer, IP addresses and DNS names are meaningless and so the MAC address is required.

===Subnet directed broadcasts===
A principal limitation of standard broadcast Wake-on-LAN is that broadcast packets are generally not routed. This prevents the technique being used in larger networks or over the Internet. [[Subnet-directed broadcast]]s (SDBs)<ref>{{cite book |chapter-url= http://rtfm.adamant.net/books/tcp-ip_illustrated/broadcas.htm |title=TCP/IP Illustrated, Volume 1: The Protocols |chapter=Chapter 12. Broadcasting and Multicasting |first=W. Richard |last=Stevens |year=2007 |archive-url=https://web.archive.org/web/20141106194636/http://rtfm.adamant.net/books/tcp-ip_illustrated/broadcas.htm |access-date=28 October 2015|archive-date=2014-11-06 }}</ref><ref>{{cite web |url=http://www.rhyshaden.com/ipadd.htm |title=IP Addressing |first=Rhys |last=Haden |work=Data Network Resource |access-date=28 October 2015}}</ref> may be used to overcome this limitation. SDB may require changes to the intermediate router configuration. SDBs are treated like unicast network packets until processed by the final (local) router. This router then broadcasts the packet using a layer-2 broadcast. This technique allows a broadcast to be initiated on a remote network but requires all intervening routers to forward the SDB.<ref>{{cite web |url=http://support.amd.com/TechDocs/20213.pdf |archiveurl=https://web.archive.org/web/20141006072000/http://support.amd.com/TechDocs/20213.pdf |title=Magic Packet Technology (White Paper, Publication# 20213, Rev: A Amendment/0) |work=AMD |date=November 1995 |access-date=28 October 2015 |archive-date=6 October 2014}}</ref><ref>{{cite web |url= https://technet.microsoft.com/en-us/library/bb632807.aspx |title=About Subnet-Directed Broadcast Wake-Up Packets for Wake On LAN |work=Microsoft System Center Configuration Manager |year=2007 |archive-url=https://web.archive.org/web/20170630165037/https://technet.microsoft.com/en-us/library/bb632807.aspx |access-date=28 October 2015|archive-date=2017-06-30 }}</ref> When preparing a network to forward SDB packets, care must be taken to filter packets so that only desired (e.g. WoL) SDB packets are permitted{{snd}}otherwise the network may become a participant in [[DDoS]] attacks such as the [[Smurf attack]].<ref>{{cite web |url=https://www.packetcoders.io/local-vs-directed-broadcasts/ |title=Local vs Directed Broadcasts |date=5 April 2018 |access-date=2023-12-07}}</ref>

===Troubleshooting magic packets===
Wake-on-LAN can be a difficult technology to implement because it requires appropriate BIOS/[[UEFI]], network interface hardware and, sometimes, operating system and router support to function reliably. In some cases, hardware may wake from one low-power state but not from others. This means that due to hardware issues the computer may be wakeable from its [[ACPI#Global states|soft off state (S5)]] but doesn't wake from sleep or hibernation or vice versa.