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From: system@LNS61.TN.CORNELL.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: re: Reply to Ethernet Address Uniqueness...
Message-ID: <9010052216.AA10852@ucbvax.Berkeley.EDU>
Date: 5 Oct 90 21:24:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 42

Bob,

>In the DEC VAX environment the unique Ethernet address on each board is
>overridden by DECNET when it starts to use that board.  The address is set
>to four bytes of a constant value plus two bytes which contain the DECNET
>area and node numbers.  Lots of opportunity for duplication!
>Does anyone know why DEC chose this scheme?

The "four constant bytes" are reserved to DEC (for DECnet)
just as the high-order butes of the physical hardware Ethernet addresses
in those Ethernet boards' address ROMs are also reserved to DEC.

In a given DECnet wide-area network, all DECnet node addresses are 
required to be unique. As a result, there is no opportunity for 
duplication on a local Ethernet either. (I am ignoring the
complications introduced by setting limits on the address ranges
passed by routers.)

One reason that DEC chose this scheme is that it allows "end-node"
DECnet systems, which have no routing information whatsoever,
to be able to send messages to other end-node systems on the same
Ethernet even when there are no DECnet routers present.
They just calculate what the Ethernet address must be and
blindly transmit the message into the ether.

Of course, the fact that DECnet addresses are limited to 16 bits
means that larger networks are limited to a maximum of about 64K
systems. My guess is that there are fewer than a dozen networks 
for which this causes problems. This address limitation in Phase IV 
DECnet is one of the reasons that DEC is moving to OSI for
DECnet Phase V.

I hope this helps.

Selden E. Ball, Jr.
(Wilson Lab's network and system manager)

Cornell University                 Voice: +1-607-255-0688 
Laboratory of Nuclear Studies        FAX: +1-607-255-8062
Wilson Synchrotron Lab            BITNET: SYSTEM@CRNLNS
Judd Falls & Dryden Road        Internet: SYSTEM@LNS61.TN.CORNELL.EDU
Ithaca, NY, USA 14853-8001   HEPnet/SPAN: LNS61::SYSTEM = 44283::SYSTEM