Digital DECnet <-> Ethernet address translation

John A. Shriver (
Mon, 7 Mar 88 10:57:41 EST

The AA:00:04:xx:xx addresses are indeed DECnet Phase IV addresses.
The low two bytes are the 16-bit DECnet node address, with the low 10
bits node, and the high 6 bits area. It is inserted in PDP-11/VAX
byte order, so it looks backwards printed Ethernet/big-indian. (All
of DECnet uses PDP-11 byte order.)

Note that they are setting the locally administered bit in the prefix
(02 in the first byte), so they are not claiming that these addresses
are globally unique. Presumably they paid their $300 for that block
from Xerox.

Well, from my understanding, the architects did not know that ARP
existed, so that option did not present itself to them. They chose
this scheme because it was simple. It is documented in "DECnet DNA
Phase IV Ethernet Data Link Functional Specification", AA-Y298A-TK.

This is not required in DECnet Phase V. Since it will use ISO CLNS as
a network protocol, it will support big long ISO addresses (12
bytes?). The little old 6 byte Ethernet address gets wrapped up
somewhere inside these big addresses, so they can use the PROM

Phase V will also eliminate their need for name-to-address translation
tables, by provide a well-designed naming service. It looks much more
flexible and dynamic than the Internet's Domain Name System.

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