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Published: Tuesday, 24 April 2007 03:05
Written by Doug Reid
Introduction Step 1 - Enable Wake on LAN Steps 2 - 5 Step 6 - Configure port forwarding Step 7 - Test again! Step 8
What is Wake On LAN? Wake on LAN, or WOL, is the ability to send a signal over a local area network (LAN) to power up a PC. Wake on WAN is the ability to send that same signal over a wide area network (WAN), such as the Internet, to trigger power-up of a PC on a private LAN.
Perhaps you have a PC at home that you don't want to leave powered on, yet may need to access while at work. Maybe you saw Al Gore's video, and you want to conserve energy and leave your PC off except when you need it. Certainly, your PC is more secure from hackers when it is off.
In order to take advantage of Wake On LAN/WAN technology, there are multiple steps to perform. This guide lists those steps, covering BIOS configurations, software, testing, routing, and security. The goal here isn't to cover every aspect of Wake On LAN/WAN technology, but to provide understanding and tools to make it work on your network.
You'll notice that testing comprises multiple steps in this guide. I can't emphasize enough the importance of making one change at a time, and then testing the effectiveness. Trust me, you'll thank yourself later for your patience and attention to detail now. I have been involved in far too many network changes that had to be completely re-done due to poor testing and methodology. If you check your work at each step, you can avoid doing it all over again later.
To find your way through all the options, it's helpful to reference the OSI model for computer networking. As you can see in Figure 1, the OSI model specifies seven layers. WOL plays primarily in Layers 1-4.
Figure 1: A chart showing the OSI model
(CCNA Intro Exam Certification Guide, Cisco Press, 2004, Wendell Odom, p34)
At Layer 1, (the Physical layer) both the PC to be turned on and the PC sending the WOL signal need to use wired Ethernet connections.
At Layer 2, (the Data Link layer) WOL uses MAC (Media Access Control) addresses to turn on a specific PC. The MAC address is critical, as a PC in the off condition doesn't have an IP address. IP addresses are loaded in memory, whereas MAC addresses are burned into the card.
The frame sent to the target PC carries a special datagram, called a "magic packet", which triggers an Ethernet card (NIC) to power up a PC. A WOL packet sent over the Internet will hit your router first. Your router will take the packet, strip the WAN IP, and send the packet to the Layer 2 MAC address on your LAN.
It's important to note that WOL doesn't work over wireless networks. Although both wired and wireless networks use MAC addressing, the format of a wireless frame is different than the format of a wired frame. Specifically, there is additional information carried in the header of a wireless frame that interferes with the magic packet frame, preventing the target machine from detecting the wakeup signal.
At Layer 3, (the Network layer) WOL works best when directed at a broadcast address. The broadcast address in most home and small business LANs ends in .255. For example, a gateway router with an IP address of 192.168.1.1 will typically have a broadcast address of 192.168.1.255.
This guide assumes your LAN uses a private IP address space, such as 10.x.x.x, 172.16.x.x, or 192.168.x.x. Typically, you'll have a gateway router providing NAT (Network Address Translation) through the public IP address provided by your ISP.
The challenge at Layer 3 (which handles IP addresses) is transmitting a Layer 2 frame, destined for a private IP broadcast address, across the Internet. Private IP addresses are not transmitted across the Internet, and broadcasts are often blocked.
The solution lies in Layer 4, the Transport layer. This layer specifies transport type (TCP or UDP) and port numbers. Communication across an IP network, the Internet included, uses both IP addresses and ports. Successfully transmitting a WOL signal over the Internet requires knowing the UDP port used by your WOL utility, and forwarding that port through your firewall.
Below are eight steps to follow to enable Wake On LAN/WAN technology on your network. Before you start, it will be helpful to have a solid understanding of your LAN and its elements. Taking the time to map and document the details of your network will come in handy. Even a small LAN of a half-dozen nodes has numerous key details, including subnet information, DHCP settings, MAC addresses, installed OS's, router configurations, etc.
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