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From: grass@unipas.fmi.uni-passau.de (Grass)
Newsgroups: comp.dcom.lans
Subject: How many physical addresses belong to 1 Ethernet station?
Message-ID: <1990Oct18.172918.24929@forwiss.uni-passau.de>
Date: 18 Oct 90 17:29:18 GMT
Sender: usenet@forwiss.uni-passau.de (USENET News System)
Organization: University of Passau, W-Germany
Lines: 35


Recently some colleagues and me discussed the above question.

As we found from the CSMA/CD ANSI/IEEE standard, there exists a *set* of
individual/multicast/broadcast addresses for each single station. Addresses
are 48 bits wide and the first bit (LSB) distinguishes individual (0) and
group (1) addresses. Group addresses may be multicast addresses (specified
by higher protocol layers!) or a particular broadcast address. This unique
broadcast address has to be predefined for the network and must be known by
every station. Usually 11...1 is chosen, but the standard leaves it open to
choose another address pattern.

This sounds that at least group address recognition is done by software: all
messages with its first destination address bit set are received by every
station, transferred to some place in memory and examined by some software
routines. If the destination address does not match with the actual set of
active group addresses the whole message is discarded from memory. What a
waste of cpu/memory/bus ressources!

To save up these ressources the Ethernet controller hardware (chip?) could
be provided with *all* addresses it shall respond to. Address decoding is
done by hardware. Only messages with according destination addresses are
passed.

Here we are back to our question: is hardware address decoding done in
practice? If yes, how many address pattern have to be buffered (adjusted by
switches??? 48-bit?) on average/maximum?

Please enlighten some curious greenhorns. Thank you in advance.

Wolf-Dieter Tiedemann

University of Passau                   email: grass@unipas.fmi.uni-passau.de
Dpt. Mathematics and Computer Science
D-8390 Passau (F R Germany)