what about 48 bits?

Steven Bellovin smb at cs.columbia.edu
Sun Apr 4 10:51:52 CDT 2010

On Apr 4, 2010, at 11:46 17AM, Jonathan Lassoff wrote:

> Excerpts from John Peach's message of Sun Apr 04 08:17:28 -0700 2010:
>> On Sun, 4 Apr 2010 11:10:56 -0400
>> David Andersen <dga at cs.cmu.edu> wrote:
>>> There are some classical cases of assigning the same MAC address to every machine in a batch, resetting the counter used to number them, etc.;  unless shown otherwise, these are likely to be errors, not accidental collisions.
>>>  -Dave
>>> On Apr 4, 2010, at 10:57 AM, jim deleskie wrote:
>>>> I've seen duplicate addresses in the wild in the past, I assume there
>>>> is some amount of reuse, even though they are suppose to be unique.
>>>> -jim
>>>> On Sun, Apr 4, 2010 at 11:53 AM, A.B. Jr. <skandor at gmail.com> wrote:
>>>>> Hi,
>>>>> Lots of traffic recently about 64 bits being too short or too long.
>>>>> What about mac addresses? Aren't they close to exhaustion? Should be. Or it
>>>>> is assumed that mac addresses are being widely reused throughout the world?
>>>>> All those low cost switches and wifi adapters DO use unique mac addresses?
>> Sun, for one, used to assign the same MAC address to every NIC in the
>> same box.
> I could see how that *could* work as long as each interface connected to
> a different LAN.
> Maybe the NICs shared a single MII/MAC sublayer somehow? I've never
> borne witness to this though.

There was a socketed ROM IC with the *machine's* MAC address on the motherboard, way back when.  If your motherboard needed replacing, the tech would move that IC to the replacement.  

Why was this done?  The reason was simple: compatibility with other stacks.  Remember that circa 1988-1990, it was not obvious that TCP/IP was going to be the winner.

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

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