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From: haas%gr@CS.UTAH.EDU (Walt Haas)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IBM TCP.
Message-ID: <8708031835.AA24234@gr.utah.edu>
Date: Mon, 3-Aug-87 14:35:58 EDT
Article-I.D.: gr.8708031835.AA24234
Posted: Mon Aug  3 14:35:58 1987
Date-Received: Tue, 4-Aug-87 04:23:44 EDT
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In article <171500006@uxc.cso.uiuc.edu>, sandrock@uxc.cso.uiuc.edu writes:
>    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)

Not globally unique by a long shot - there can be only 65534 DECnet addresses
by this number system.

However there is an even more embarrassing problem with this arrangement-
(and this problem also occurs with XNS which does the same thing).
Suppose you have a DECnet (XNS) host which is connected to two Ethernets.
Now each DECnet (XNS) node is allowed only a single DECnet (XNS) address,
which the software loads into [all of] the Ethernet interface[s] attached
to the node.  This means that the node has the same 48-bit Ethernet
address on both Ethernets.  OK, now what happens when you attach  a
learning Ethernet bridge, such as the DEC LANbridge 100 or any of the
numerous competitors between the same pair of Ethernets?  Well, what
happens is that when the node sends a packet on one Ethernet, the bridge
learns the node is on that net, and starts routing all packets with
that destination to the net where it saw the node's address.  Then the
same node sends a packet on the other net, using the same Ethernet
source address, and the bridge says "whoops, that node just moved"
and commences to route all packets for the node across to the other
Ethernet!


----------------*
Cheers  -- Walt     ARPA: haas@cs.utah.edu     uucp: ...utah-cs!haas

"...The unforgivable but by no means uncommited sin in this connection occurs
 when, A calling down that his rope is caught, it devolves that B is himself
 standing upon it.  Any second man once guilty of this precious bit deserves
 to be demoted from his position for the remainder of the season!"

   Robert L. M. Underhill, "ON THE USE AND MANAGEMENT OF THE ROPE
        IN ROCK WORK," Sierra Club Bulletin, February 1931, p. 80.