How to turn on pc remotely (Wake On Lan)

[badass]

Are you sure about that? I think most NAT routers just drop the packet if they don't know where to send it. On my old router (a linksys befsr41) I had to forward an external port to the an internal broadcast address (192.168.1.255) to get a packet sent to all ports. My new router (a linksys WRT54G) doesn't support forwarding to a broadcast address and I've been unable to get WOL working from outside the LAN.

If a router has an IP address, but no MAC address it will broadcast an ARP request to all ports it has. However across the internet with WOL, things are different. In this case, the router has an IP address and even though the magic packet has the MAC address, the router doews not know this. The router will send out an ARP request (what is the MAC address for this IP address) and the computer will not respond since it is off. Since the router cannot find the hardware address for the IP address it has, it may well drop the packet.
For WOL to work in this case, you need it to be sent from the local subnet, or do what you did. Send it to a directed broadcast address.

[ajbrun]

So instead of sening it to a specific internal IP, you need to send it to an internal IP ending in 255 for it to send it to all ports? That will then work, even if the computer has been off for a length of time?

[badass]

So instead of sening it to a specific internal IP, you need to send it to an internal IP ending in 255 for it to send it to all ports? That will then work, even if the computer has been off for a length of time?

Not quite. A broadcast address does not necessarily end with 255
If your router supports it, then you can set up a port mapping that sends traffic for a certain port to your internal networks broadcast address.
Post one of your internal IP addreses here with the subnet mask and I'll tell you the broadcast address.

[ajbrun]

Do you not just take the last part of the routers IP address, and replace it with the last part of the subnet mask?

I'd prefer you explained how the broadcast address is found rather than just telling me .

[badass]

Do you not just take the last part of the routers IP address, and replace it with the last part of the subnet mask?

I'd prefer you explained how the broadcast address is found rather than just telling me .

You like binary maths?
OK then to get the broadcast and network addresses form an IP address and subnet mask:
take the IP address and convery to binary.
Take the subnet mask and convert to binary You will see the subnet mask is a load of 1's followed by a load of 0's. This is used to tell your TCP/IP stack which part of teh address is the network address and which part is teh node address. The IP address will be 1's and 0's in no apparent order.
Perform a logical AND for each bit. The end result will be your network address.

example

10.2.3.4=00001010.00000010.00000011.00000100
255.255.255.224=11111111.11111111.11111111.11100000
00001010.00000010.00000011.00000100 AND
11111111.11111111.11111111.11100000 =
00001010.00000010.00000011.00000000

Convert back to decimal and you get the network address of 10.2.3.0
OBVIOUS! you think. Now lets get the broadcast address. The subnet mask tells us that the first 26 bits are the network address. The broadcast address is the last address in that network so it will have the same network address, but the node address will be all 1's
so the broadcase address is 00001010.00000010.00000011.00011111 or 10.2.3.31

However if your subnet mask is 255.255.255.0 then the broadcast address will always end with .255 as the last 8 bits (referred to as an Octet in IP terminology) are all 0's
Because subnet masks are always a load of 1's followed by 0's, a lot of people these days write IP addresses/subnet masks as 10.2.3.4/26 insted of 10.2.3.4/255.255.255.0 - The first is the Cisco way, the second is the Misrosoft way.

[ajbrun]

When I do ipconfig, I get a different subnet mask to what's shown on my router. Any idea why? One ends in .0 and the other in .255.

Also, which bit's the node address?

[badass]

When I do ipconfig, I get a different subnet mask to what's shown on my router. Any idea why? One ends in .0 and the other in .255.

Also, which bit's the node address?

If your router and computers subnet masks differ, one of them is wrong.
My money's on the one ending .255
enter here the full IP addresses of your router and computer along with their subnet masks etc and i'll tell you whats up.

The node address is the part of the ip address that is not the network address - i.e. the last bit.

[ultim8um]

http://jodies.de/ipcalc

You might find that helpful...