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From: rpk@mit-eddie.UUCP (Robert Krajewski)
Newsgroups: net.lan
Subject: Ethernet II ARP and subnets
Message-ID: <1859@mit-eddie.UUCP>
Date: Wed, 16-May-84 15:19:01 EDT
Article-I.D.: mit-eddi.1859
Posted: Wed May 16 15:19:01 1984
Date-Received: Thu, 17-May-84 04:23:19 EDT
Organization: MIT, Cambridge, MA
Lines: 47


	Newsgroups: net.lan
	Message-ID: <3316@fortune.UUCP>
	Date: Tue, 15-May-84 19:05:52 EDT

If you are just buying controllers from some vendor such as 3Com or Interlan,
not to worry. The controller comes with a default address which is (supposed
to be) unique, set by the vendor.

	Note: If you are going to be a gateway host between two
	Ethernets, make sure your controllers can have the address
	overridden by host software, since it is very important that
	the same absolute physical address appear on both nets. (Usually,
	you go read the default addresses from all the controllers, pick
	one [the smallest? the "first"?], and write that to all of them.)

This is probably not the right thing to do, especially when you considered
that most major protocols will have their own software addresses for host
which will not be trivially and correctly translatable into Ethernet
hardware addresses.  To interface Ethernet to a higher level protocol, one
should look at Dave Plummer's Address Resolution Protocol, which is
becoming a de facto standard.  Like someone said before on this list,
you've got to be smart if you have a gateway to another network (not
neccessarily an Ethernet).

For example:

Chaosnet is a local area network that was first both hardware and software,
but now can run with Ethernet II hardware.  At MIT, both old Chaosnet cable
and new Ethernet cable are subnets of the entire MIT Chaosnet.  (Note that
the Ethernets can also carry IP packets, which the pdp11 Chaosnet bridges
ignore entirely.)  So the Chaosnet hosts on the Ethernet need to fill in
their hardware destination address in the Ethernet frames, and it does
matter if the actual destination host has an Ethernet interface or an old
Chaosnet interface.  The packet has got to get to the bridge, which will
know what to do with it when it looks at the higher-level Chaosnet address.
[The parallel situatiuon in the Internet world is when J. Random
Workstation is confronted with sending a IP datagram to, say, Stanford.]
The general guideline is for the gateway respond with its own Ethernet
hardware address when it gets any ARP request for a host not on the subnet
to which it is connected.  How the packet actually gets routing will be
determined by the higher-level protocols involved.
-- 
``Bob'' (Robert P. Krajewski)
ARPA:		RpK@MC
MIT Local:	RpK@OZ
UUCP:		genradbo!miteddie!rpk
	or	genradbo!miteddie!mitvax!rpk