The tale of a single MAC

Mark Smith nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org
Sat Jan 1 22:33:24 CST 2011


On Sat, 01 Jan 2011 20:59:16 -0700
Brielle Bruns <bruns at 2mbit.com> wrote:

> On 1/1/11 8:33 PM, Graham Wooden wrote:
> > So ­ here is the interesting part... Both servers are HP Proliant DL380 G4s,
> > and both of their NIC1 and NIC2 MACs addresses are exactly the same.  Not
> > spoofd and the OS drivers are not mucking with them ... They¹re burned-in ­
> > I triple checked them in their respective BIOS screen.  I acquired these two
> > machines at different times and both were from the grey market.  The ³What
> > the ...² is sitting fresh in my mind ...  How can this be?
> 
> 
>  From the same grey market supplier?
> 
> I know HP has a disc they put out which updates all the firmware/bios in 
> a specific server model, its not too far fetched that a vendor might 
> have a modified version that also either purposely or accidentally 
> changes the MAC address.  Off the top of my head, I'm not sure where the 
> MAC is stored - maybe an eeprom or a portion of the bios flash.  Or, it 
> could be botched flashing that blew away the portion of memory where 
> that was stored and the system defaulted to a built in value.
> 
> Excellent example is, IIRC, the older sparc stuff, where the ethernet 
> cards didn't have MAC addresses as part of the card, but were stored in 
> non-volatile or battery backed memory.

This was actually the intended way to use "MAC" addresses, to used as
host addresses rather than as individual interface addresses, according
to the following paper -

"48-bit Absolute Internet and Ethernet Host Numbers"
Yogan K. Dalal and Robert S. Printis, July 1981
http://ethernethistory.typepad.com/papers/HostNumbers.pdf

That paper also discusses why 48 bits were chosen as the size, despite
"Ethernet systems" being limited to 1024 hosts. 

I think things evolved into MAC per NIC because when add-in NICs
were invented there wasn't any appropriate non-volatile storage on the
host to store the address. 

Regards,
mark.




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