The tale of a single MAC

Steven Bellovin smb at cs.columbia.edu
Sun Jan 2 22:38:54 UTC 2011


On Jan 2, 2011, at 5:15 54PM, Mark Smith wrote:

> Hi,
> 
> On Sun, 2 Jan 2011 08:50:42 -0500
> Steven Bellovin <smb at cs.columbia.edu> wrote:
> 
>> 
>> On Jan 1, 2011, at 11:33 24PM, Mark Smith wrote:
>> 
>>> 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:
> 
> <snip>
> 
>>>> 
>>>> 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
>> 
>> Yup.
>>> 
>>> 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. 
>>> 
>> On really old Sun gear, the MAC address was stored on a separate ROM chip; if the
>> motherboard was replaced, you'd just move the ROM chip to the new board.
>> 
>> I'm not sure what you mean, though, when you say "when add-in NICs were
>> invented" -- the Ethernet cards I used in 1982 plugged into Unibus slots
>> on our VAXen, so that goes back quite a ways...
>> 
> 
> More that as add-in cards supplied their own "storage" for the MAC
> address, rather than expecting it from the host (e.g. something like
> MAC addresses set by init scripts at boot or the ROM chip you
> mentioned on Suns), this has now evolved into an expected model of a
> MAC address tightly bound to an Ethernet interface and supplied by the
> Ethernet interface e.g. by an add-in board if one is added. Now that
> this model as been around for a long time, people find it a bit strange
> when MAC addresses aren't as tightly bound to a NIC/Ethernet interface.
> This is all speculation on my part though, I'd be curious if the
> reasons are different.
> 
> When I first read that paper, it was really quite surprising that "MAC"
> addresses were designed to be more general host addresses/identifiers
> that were also to be used as Ethernet addresses. One example they talk
> about is using them as unique host identifiers when sharing files via
> floppy disk.
> 
If you read the XNS specs, you'll see that they liked 64-bit addresses --
a 16-bit network number and a 48-bit host address.  In other words, they
had id/locator separation...


		--Steve Bellovin, http://www.cs.columbia.edu/~smb









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