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From: DCP@QUABBIN.SCRC.SYMBOLICS.COM (David C. Plummer)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IBM TCP.
Message-ID: <870803092021.7.DCP@KOYAANISQATSI.S4CC.Symbolics.COM>
Date: Mon, 3-Aug-87 09:20:00 EDT
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Posted: Mon Aug  3 09:20:00 1987
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    Date: 31 Jul 87 16:30:00 GMT
    From: uxc.cso.uiuc.edu!sandrock@a.cs.uiuc.edu

    DECnet uses the node address to set the least significant 16 bits of the
    48-bit Ethernet hardware address while setting the most significant 32
    bits to a "known" constant value.  Specifically, the Ethernet address
    will be AA-00-04-00-xx-xx, where the xx-xx fields are the DECnet node
    address (area-number * 1024) + node-number.
      There may be both certain advantages and also disadvantages to this
    approach, but is it true that these addresses are not globally unique?

      Mark Sandrock, (sandrock@uxc.cso.uiuc.edu)

The DECnet scheme "only" allows 64K hosts, or probably more precisly,
only 64 areas.  I would be quite surprised if there are only 64
installations of DECnet in the world.  Therefore, if we assume that
people conventionally number node within an area starting with 1, then
there is likely a global non-uniqueness between machines of two
installations that have the same area number.  I have no idea how DECnet
area numbers are assigned.

The major disadvantage is that if ACME Computers (read: some other
vendor) also used an algorithmic approach, it would be impossible, 100%
impossible, for a machine to be able to support both protocols at once
on one transciever, since it would require the transceiver to have two
Ethernet addresses.  Such multi-protocol machines do exist.  Luckily,
there is only one vendor that I know of (DEC) that uses an algorithmic
Ethernet address.  I'm pretty sure DEC knew about ARP in time; I'd have
to get some ancient mail files off of tape to make sure.