What triggers DHCP?

[sarta]

I have a Dell laptop that I can turn on by sending a magic packet from a Lenovo laptop. (using a cat5 cable connected between the built-in ethernet ports on both computers)

When the Dell is OFF and I plug the cat5 cable into my Lenovo, the Lenovo's network icon indicates that it is attempting DHCP, which fails and results in the Lenovo getting an autoconfiguration IP, then I can send the magic packet to the Dell.

Now I'm trying to wake up the Dell from a magic packet from a trendnet usb-to-ethernet port on the Lenovo but it's not working. When plugging the Dell into the Lenovo that way, the usb-to-ethernet port does not attempt DHCP but instead stays in the "media disconnected" state.

My question is how does the Lenovo's builtin ethernet port sense the ethernet connection to the Dell (which is off), and how can I enable the usb-ethernet port to likewise detect the connection and attempt DHCP?

 

[Torturednacho]

I guess I'm not understand how DHCP even fits into this?
If there was a network someone in there then it would be Wake-on-Lan. Maybe ICS? .. But Then again, I have no clue how DHCP even works in your question?

 

[sarta]

Thanks for your reply.

Basically when I plug the cable into the Lenovo, I see that my network connection is detecting some kind of signal on the cat5 cable that is plugged into the Dell which is powered off (but still listening for magic packet), because the Lenovo starts attempting DHCP when I plug the cable in, then it times out and uses an autoconfiguration IP. I want to enable my usb-ethernet adaptor to also recognize the signal from the Dell, so that it will attempt DHCP.

 

[SimonDavies]

you're using a cable to just connect between the two machines? is this a normal or crossover cable? If it's a normal cable then you 'shouldn't' be able to talk directly to the other laptop anyway. If it's a crossover cable then you need to assign a static address to the Lenovo (same address range as the Dell).

For DHCP to work you need to have a DHCP server located somewhere on the network, if you just have the two laptops then the chances are you aren't running DHCP on either of them.

DHCP servers usually run on either a server OS or router.

As far as magic packet goes, it will be targeting the MAC address of the machine rather than the IP address of it (after all it's off, so it won't have an ip address), as far as APIPA goes (the address that you're getting) you should be able to send the UDP packet from there to the Dell, it's when the Dell powers up and uses an ip address that the Lenovo doesn't understand that you would have problems.

If you are just trying this between the two machines with no switches\hubs in between then the first thing you should be using is a crossover cable, secondly you should assign static addresses (192.168.1.x etc).

Simon

The real world is not about exam scores, it's about ability.

 

[tlcscousin]

media disconnected state"
That pretty well sums up the issue if it shows media disconnected the NIC does not detect and kind of ethernet cable so windows doesnt try to do anything with the card.
That could be drivers it could be media sensing and it could be duplexing. When you get the other card to show a connection then it should work exactly the same as the first card.

 

[SimonDavies]

not if he is plugging a straight through cable directly into another nic without a switch in between.

Simon

The real world is not about exam scores, it's about ability.

 

[burtsbees]

I want to enable my usb-ethernet adaptor to also recognize the signal from the Dell, so that it will attempt DHCP."

Nope, won't work. The ethernet is always powered by 5VDC, even when the comp is "off". This is the reason wake-on-LAN works. You can see ethernet activity (link lights) on an ethernet port when a server (specifically servers) is off.

USB does NOT have power going through constantly.

HTH

/

 

[burtsbees]

BTW, when a comp is set for DHCP, it sends a DHCP-REQ packet, the server sends a DHCP-ACK, the comp a DHCP-ACK-ACK, server DHCP-OFFER, pc DHCP-THANKS...

something like that...lol

Been a while, but the DHCP-REQ and DHCP-OFFER packets are not made up...lol

/

 

[sarta]

Found the problem. The port that worked was a gigabit port which had auto mdi/mdx, and the other one that didn't work didn't have the auto mdi/mdx, so it needed a crossover cable to work right.

 

[SimonDavies]

Pretty sure that's what I said in my first sentence.

Simon

The real world is not about exam scores, it's about ability.