Re: IBM TCP. Really DECnet and Ethernet addresses

Rich (RMRichardson.PA@Xerox.COM)
11 Aug 87 19:02:33 PDT (Tuesday)

>I guess I'm hopelessly out of date.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...

I don't have a copy of the blue book, but the relevant section in IEEE 802.3 is 3.2.3 (with Fig 3-2) on pages 24, 25 of the IEEE book (green cover).

> Along slightly different lines, I fail to see how the global
> ethernet addressing scheme, whether managed as a flat address space
> or in some hierarchical fashion, hasn't blown up yet. I mean there
> just aren't that many addresses available. Just as an isolated
> example, what happens to ethernet addresses on failing boards?
> We've probably had the ethernet board on one of our -20's replaced
> three times already. Am I to believe that DEC Field Service
> records these hardware addresses, and when bad boards come back
> that can't be repaired, they tell some global address administrator
> that this particular hardware address has now been freed and can be
> used again?

Given that 2 of the 48 bits are used by the "flags," that leaves 2**46 addresses for ethernet devices. This is a large number that is hard to get hold of until one reduces it to reasonable. Conversion to a power of ten is a start: 7.036874e+13. Even 10**13 is a very large number to get hold of; lets suppose we can produce one thousand devices every second, round the clock, until the space is full. 7.036874e+13, /60 secs/min, /60 min/hour, /24 hours/day, and /365.25 days/year. 2229.851 years!

Even the space allowed for a single manufacturer is 2**24 or about 16 million addresses. (And of course if some manufacturer fills out that space, the IEEE can allocate a second block of 16 million. :-)

I don't know how DEC Field Service takes care of the problem of bad boards, but when we have to change the relevant board in one of the D-machines around here, we pull the PROM which contains the ethernet ID out of the old board and stick it in the replacement so our machine is as it always was to the ethernet. Only if you break the PROM do you get a new number and go through the pain of re-registering your machine on the net.


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