Re: IBM TCP. Really DECnet and Ethernet addresses

David C. Plummer (DCP@QUABBIN.SCRC.Symbolics.COM)
Tue, 11 Aug 87 10:38 EDT

There are a lot more Ethernet addresses than Internet addresses, and I
don't hear people screaming (yet) that we're running out of Internet
addresses. (I do remember screaming that there weren't enough class A
subnets, and later screaming that we had to be careful about doling out
class B and C addresses because of fixed-size non-replacable tables in
some older gateway implementations. Anyway...) The V1.0 Ethernet
allowed for 2^23 vendors. That's 8 million vendors. Each vendor gets
2^24 addresses. That's 16 million addresses. It's conceivable that a
vendor could run out of addresses, and my solution would be to give the
vendor another (or several more) chunks of 2^24 addresses. Even if
every person on the planet (about 2^32 people) had 64 machines (2^6) and
got a new address for each machine every year, the Ethernet address
space (2^47) allows for 2^(47-32-6) = 2^9 = 512 years of such
(farfetched) activity. This is why the addressing hasn't "blown up
yet," to use your phrase. There is therefore no need to recycle
hardware addresses if a particular board (e.g., your DEC-20's) is

As for your DEC-20 board, replacing a board and/or changing an Ethernet
hardware address for a host works pretty well if you use a
dynamic protocol->hardware address translation mechanism such as ARP.
The minor problem being the transition time until the hosts learn of the
new address, and this problem is discussed in the RFC.

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