Re: IBM TCP. Really DECnet and Ethernet addresses


David C. Plummer (DCP@QUABBIN.SCRC.Symbolics.COM)
Mon, 10 Aug 87 10:10 EDT


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

    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.

I guess I'm hopelessly out of date. My copy of the Blue Book is Version
1.0, Sepetmber 30, 1980. In it, in section 6.2.1 on page 21, there is
no mention of this second bit being for local/global administration. In
fact, of the physical address it says
        "A station's physical address should be distinct from the
        physical address of any other station on @i(any) Ethernet."
The italics are in the book. Time marches on... standards change...



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