[ih] Re: IPv4 vs MAC

Craig Milo Rogers rogers at ISI.EDU
Sun Jul 29 16:39:08 PDT 2001

>On Fri, Jul 27, 2001 at 03:40:53PM +0700, Robert Elz wrote:
>> Does no-one else still recall the world before ARP, where MAC addresses
>> (ie: the old 16 bit things that 3Mbit ethernet used) were embedded in the
>> bottom 16 bits of your class B address (no-one had anything smaller than
>> that of course...)

	The 3-Mbit Xerox PARC Ethernet packet format has 8-bit wide
address fields, not 16-bit wide ones.

	At ISI, I connected our first 3-Mbit Ethernet to the ARPAnet
via PDP-11 running the EPOS operating system.  Our Ethernet addresses
were mapped into IP network 10.  Due to a pecularity of network 10
addressing (due, in turn, to a convention of the expanded IMP
addressing scheme, if I recall correctly), our 3-Mbit Ethernet
addresses actually mapped into the third octet of the IP Net 10
address, rather than the 4th octet.  However, this mapping was
transparent to other Net 10 hosts, so the ISI gateway didn't have to
participate in IP-level routing protocols (which considerably
simplified its construction and maintenance).

	We installed a second ARPAnet-to-Ethernet gateway at DARPA.
The primary purpose of the gateways was to provide Internet access to
the Xerox "Penguin" laser printer that had replaced the XGP (Xerox
Graphic Printer) printer at each site.  The printers used a Xerox Alto
as a front-end to the actual print engine, which was a highly modified
Xerox copier.  Our gateways performed an application-level
transformation between the Internet TFTP protocol and the Xerox EFTP
protocol; the two protocols were very similar, and the transformation
was nearly stateless.

	Dave Mill's "fuzzball" system provided similar
Internet-to-3Mbit-Ethernet gateway services, as well as a variety of
other utility services, and ran on less expensive PDP-11
configurations than EPOS.  For a while, there were quite a few of them
installed around the Internet.

					Craig Milo Rogers

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