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From: normt@ihlpa.UUCP (N. R Tiedemann)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet Physical Address
Message-ID: <2316@ihlpa.UUCP>
Date: Mon, 17-Nov-86 17:21:34 EST
Article-I.D.: ihlpa.2316
Posted: Mon Nov 17 17:21:34 1986
Date-Received: Tue, 18-Nov-86 07:44:53 EST
References: <939@ssc-vax.UUCP>
Distribution: na
Organization: AT&T Bell Labs, Naperville, IL
Lines: 34

> 
>  I have always been under the assumption that Ethernet Physical Addresses
>  were virtually "cast in concrete" , ie unchangable.  I had heard that
>  Xerox maintains a registry of physical addresses, and that manufacturers may
>  ...
>  replaces it with a physical address like aa-00-04-00-25-04, where the "25"
> 				Bruce E. Stock
> 				uw-beaver!ssc-vax!bruce

Xerox does keep a registry (soon to be taken over by IEEE) of "all" ethernet
physical addresses. The thing to not about this address change is the "aa" in
the first byte. Normally the first 3 bytes are the manufacture's code and the
last 3 are the device.

Now, everyone knows that the first bit (LSB) of the first byte specifies a
group address (vs. a physical address), but what is not know by many is the second
bit is a local/global administered bit. If that bit is set, the device is not
registered with Xerox, and there is a possiblity of two devices having the
same physical address. This is normally used in closed systems, where outside
devices are not attached. (Restricted LAN). So once the network is set-up the
individual devices can "change" their physical addresses, and set this bit
and be "legal". Most often this is used where a network is initialized and not
changed, i.e. no new nodes added or dropped. We use this when we use "ethernet"
to communicate between processors in a multiprocessor system. No one else is
supposed to be connected to this network, it doesn't go outside of a couple of
frames of equipment, and we may build lots of them, but they are NEVER 
interconnected.


		Norm Tiedemann
		ihnp4!ihlpa!normt
		AT&T Bell Labs

"For a good time, read an IEEE standards spec."