The difference between PXE and Wake-on-LAN (WoL)

Basically, when your machine is plugged in, even turned off, some maintain some very basic functions such as the network card.

They can be configured so that when a special instruction is received, it can power up the machine.

The easiest way to start is simply, if you have an integrated NIC, look in the BIOS and see if it supports it. If you have an actual Network card, you may not be able to use it... But it is worth seeing if you have a slot for a Wake On Lan Chip or cable

Also, as for saving energy, you need to obviously turn on your device! You need to have a router that can turn on devices based on a schedule or a wireless mobile or similar that you can send the instruction from - obviously though, you may just be better off turning the machine on via its power button!

Wikipedia has a good article on Wake On Lan

Wake on LAN (WoL) support is implemented on the motherboard of a computer and the network interface, and as such, is not dependent on the operating system running on the hardware, although the operating system can sometimes control the WoL behaviour. If the network interface is a plug-in card rather than being integrated into the motherboard, the card may need to be connected to the motherboard by a cable. Motherboards with an embedded Ethernet controller which supports WoL do not need a cable.

WoL works by sending a specially formatted ethernet packet to the MAC address of your computer. If you are on the local network (the LAN) you can communicate direct with your computer's MAC address and WoL works all the time quite happily.

From anywhere outside the local network you rely on IP routing to forward IP encapsulated packets through the network to get to your computer. This forwarding relies on ARP (Address Resolution Protocol) at the final hop (your router to your computer).

Your router keeps a record of which IP addresses are mapped to which MAC addresses - the ARP table - which periodically has stale entries purged.

Under normal network operation the router first sends an ARP WHOHAS packet to all the computers on the network (MAC address ff:ff:ff:ff:ff:ff) requesting the computer that has the IP address specified in the packet to respond with an ARP HAS packet stating that it has the IP address in question. This gets cached in the ARP table for a period of time.

If the computer is switched (or asleep) the network stack can no longer respond with anything, so the ARP WHOHAS request never succeeds - so the router cannot work out where to send the packet to - hence WoL then fails.

There are 2 possible ways around this problem:

• Set up a static ARP entry on the router

This is by far the simplest if the router has the ability to store a MAC->IP mapping permanantly. This will remove the need for ARP to try and discover the MAC address of your computer.

• Have another network device on the local network that never sleeps and is able to perform the WoL for you.

Seems to defeat the object really, but it may be the only way to do remote-WoL.