Generic MAC Address Problem

[Lee_SW]

Hi,

Today I was tasked with troubleshooting a pc that would not connect to
the internet.

The PC will get the the 192.x.x.x IP address and then eventually drop out to a 169.x.x.x IP address.

Connecting the pc to a neighbours router allowed internet access, thus eliminating the PC being at fault.

On spekaing to Virgin Tech Support we were eventually told that the problem was with our network card having a "generic MAC address".
Because their servers had already issued an IP address to another network card with our MAC address it wouldn't issue another.

Basically they are saying our network card has the same MAC address as someone else's network card.

I thought they issued their IP addresses based on the Modem's MAC address?

I am curious to hear others simular experiences or any Tech insight into this alleged issue!


Thanks

L

[rossco555]

Thats total twod.....every network device has its own UNIQUE MAC address!

[Hugh]

Unless it's been cloned......

[TraxData]

which most have...

[kaydee]

Surely the server gives an external IP to the router and the router gives an internal IP to the network card (without the need to ask the server), regardless if there is an internet connection or not? Thus the MAC address of the network card makes NO difference what so ever?

[dev]

Thats total twod.....every network device has its own UNIQUE MAC address!

in theory yes, in practice, no!

[Mr Clean]

Try changing the mac address on the comp. ;)

http://tmac.technitium.com/tmac/index.html

It nice and easy. Just change the last figure/letter of your existing mac address to one higher or lower than the value. Reboot modem and Hey Presto. Should have connectivity.

If not, there's some other issue.

[Tech_Boy]

If using the generic windows drivers for your ethernet card then it is very likely that you would have a mac address issue, as the windows drivers seem to get the nic to report a different mac address than if you were using the specific drivers for your make & model of nic.

[Bill C]

Thats total twod.....every network device has its own UNIQUE MAC address!

Wrong. We had 8000 usb adapters sent to us in the early days of STB broadband with the same mac address. It has happened and it will happen again.

And just to top that i have 2 M2N-E SLI motherboards in 2 of the machines in my house and they both had the same mac address. I had to change them in the BIOS.

[Toto]

Hi,

Today I was tasked with troubleshooting a pc that would not connect to
the internet.

The PC will get the the 192.x.x.x IP address and then eventually drop out to a 169.x.x.x IP address.

Connecting the pc to a neighbours router allowed internet access, thus eliminating the PC being at fault.

On spekaing to Virgin Tech Support we were eventually told that the problem was with our network card having a "generic MAC address".
Because their servers had already issued an IP address to another network card with our MAC address it wouldn't issue another.

Basically they are saying our network card has the same MAC address as someone else's network card.

I thought they issued their IP addresses based on the Modem's MAC address?

I am curious to hear others simular experiences or any Tech insight into this alleged issue!


Thanks

L

There have been instances of network card vendours making errors in the MAC allocation. It is true though, your IP address is based on your client MAC address, not the modem one.

So you can get issues where two clients on the same uBR have the same MAC address, but this can usually be ok as long as the modem MAC addresses are different.

EDIT - See Bill C knows, a little digging around can prove that. :)

Unless it's been cloned......

Possibly, no evidence though.

which most have...

Most!...evidence please, VM has about a few million BB customers.

[sollp]

Just a point on this one, i was using windows 98 and XP on 2 different lappys. This has happened ages ago when using 98, when using same modem and swapping between 2 lap tops i have noticed it gives the 169 IP on the windows 98 and a reboot normally sorts it out. Think its because the modem has given IP to one machine and the windows 98 needs a reboot when swapping on same modem. If you get my explanation.

[Incomplete]

Basically they are saying our network card has the same MAC address as someone else's network card.

I thought they issued their IP addresses based on the Modem's MAC address?

There are 2 DHCP transactions involved. One is to provide the modem an IP address, which is transparent to the client and is purely for management and addressing of the modem, the other is to provide a client IP address, which is assigned to a MAC address.

Each DHCP transaction is based on a map of MAC address to IP address. When the uBR wants to talk to a PC rather than arping for its' IP address (also broken by matching MACs) it will perform a DHCP query.

When your PC requests an IP address a record is made of the MAC of the modem that the request passes through, for purposes of mapping that to a SFID on the cable modem downstream, however the actual DHCP lease is provided to CPE MAC address, not cable modem.

*phew*