can't get a valid IP from dhcp

[pirpir]

hi
i have a laptop with xp pro. when I use my laptop at school, and plug in the rj45 cable, my ethernet card obtains an IP properly from dhcp. However, when I try to do the same thing at home with my cable modem, I can't get a valid IP from the dhcp server. There's nothing wrong with my modem/cable or cable company since when I connect the same cable to my 98 desktop computer, it works.
I can get a valid IP if I connect using USB but I should be able to do so by using the ethernet card directly.
help desk operators from my cable company said there was a difference between getting an IP from dhcp at school ang getting it from dhcp at home and they told me to reinstall my driver, which didn't work..
if anyone knows why this happens, i will be glad..
thanks anyway..

 

[bcastner]

I have seen issues due to this:

http://support.microsoft.com/default.aspx?scid=kb;en-us;167014

Open notepad and copy/paste the below as dhcp_reset.cmd

netsh int ip set address "Local Area Connection" dhcp
ipconfig /release
ipconfig /renew

 

[pirpir]

I had already tried releasing and renewing the IP address from the command line but it says 'error trying to renew IP address' and doesn't renew it..

and the link you suggested is concered about the cases where there are two dhcp servers. Mine is like that too, but I used to get the same error before taking my laptop to school- the problem was present before I introduced the second dhcp server..

so I still don't know what to do..

 

[bcastner]

You might have an autonegotiation issue between your ethernet card and the modem. Try placing an inexpensive switch between the two devices, by connecting the modem and the pc to the switch instead of directly.

 

[pirpir]

ok I'll try that
thank you

 

[dearingkr]

I've had this problem before.

It seems to be something to do with the ISP seeing changing MAC addresses.

When I first put my router on my network after having a PC on there for a while, the router wouldn't get an IP until I cloned the MAC address of the PC that was on previously.

I later found that removing power from the cable modem for a few minutes then bringing it back up with the new device solved the issue.

MCSE CCNA CCDA

 

[bcastner]

dearingkr,

That is a great point to make. Most cable modems (even those with an on/off switch) should be unplugged (give it one full minute) if you are switching workstations behind them. As dearingkr notes, the MAC address of the ethernet card is recorded for authentication. In some cable systems you need to use an online wizard to record a possible new MAC address, or call their Tech Support line.

Most residential grade routers offer a MAC "clone" feature to avoid issues. You can enter the MAC address of your original machine and the router will present that MAC to the cable system.

You can find the MAC address of the ethernet card by doing: Start, Run, CMD
Ipconfig /All

Look for the entry "Physcial Address"

 

[AvayaNovice]

Hi,

You'll need to clone your MAC address, because the cable modem has locked to the MAC of the other machine, and regardless of rebooting the modem... some cable operators lock it permanantly to that computer until you call them and tell them otherwise...

 

[pirpir]

I know that rebooting the modem doesn't work, but i have no idea what you mean by saying "cloning the mac address".. how do I clone it, where do I clone it?
and just out of curiosity, why doesn't the same problem occur when I connect the USB cable?

 

[bcastner]

To clone a MAC address, you would need an additional piece of equipment: a broadband router.

As to why the USB works? Same thing, I suspect, that you have registered whatever is returned from the USB interface (very different from the ethernet interface) as the MAC address.

 

[AvayaNovice]

Yes, the USB port has a different MAC address than the EtherNet port, but it should still lock to only one... regardless of medium.

Call your cable company. When you talk to tech support, ask them if they can see a device attached to the modem, most UBRs can detect this as it is part of DOCSIS compliancy. If they can... then something's up on your end, if not, they should be able to reboot your modem remotely and unlock its MAC authorization table.

 

[niklasw]

ipconfig/flushdns


-Cheers, Niklas

 

[YellowOnline]

This is an example of a router config. You can see where you can clone the MAC-ID.

Peace,

Yellow