Re: IBM TCP. Really DECnet and Ethernet addresses


ERCF02 Adam Hamilton (mcvax!ukc!its63b!adam@seismo.css.gov)
7 Aug 87 14:00:44 GMT


> Date: Wed, 5 Aug 87 11:24:18 CDT
> From: sandrock@uxc.cso.uiuc.edu (Mark Sandrock)
>
> It sounds like you are claiming that DEC preempts Ethernet hardware
> addresses which have been officially assigned to other vendors, but
> I have yet to see a specific example of such an address. Is it asking
> to much either to see an example or else have everyone drop this claim?
>
>Sure. There is a Symbolics 3600 at our site that has an Ethernet
>hardware address of 08-00-05-03-00-38. The 08-00-05- portion was
>assigned to Symbolics by the Ethernet number Czar for use by Symbolics
>for its Ethernet interfaces. That is the "official" hardware address of
>the interface on that machine. That machine also has a DNA address of
>41.69. Because of the way DNA works, the booting procedure for the
>machine changes the Ethernet address of the interface to
>AA-00-04-00-45-A4. Therefore, DNA has preempted our officially assigned
>address.

In IEEE802 and also XEROX Blue book, the format of an Ethernet address
is defined as having two special bits; these are the first two to be
transmitted. Note that bits in an octet are transmitted
LEAST significant bit first, therefore these bits are the LEAST
significant bits of the first octet.
        The first bit signifies whether the address is individual or local,
the second signifies whether the address is globally (value 0) or locally
administered. In the above example (AA-00- etc.) the second bit is set,
therefore the address is locally administered.
        All address allocations to manufacturers are globally administered.
All VMS machines I have seen have addresses which start AA-00. All this
means that DECnet style addresses are NOT globally unique (as discussed)
but do NOT use values which are globally administered. This should make no
difference unless the same address is used on more than one Ethernet
when a bridge (specifically selective frame-level repeater) may well get
confused.
        Presumably this bit was put in for DEC's benefit, but I'm just
guessing here.



This archive was generated by hypermail 2.0b3 on Thu Mar 09 2000 - 14:38:49 GMT