IPv4 address length technical design

Jeroen van Aart jeroen at mompl.net
Tue Oct 30 00:54:53 UTC 2012

On 10/03/2012 09:52 AM, Seth Mos wrote:
> Op 3-10-2012 18:33, Kevin Broderick schreef:
>> I'll add that in the mid-90's, in a University Of Washington lecture
>> hall, Vint Cerf expressed some regret over going with 32 bits. Chuckle
>> worthy and at the time, and a fond memory
>> - K
> "Pick a number between this and that." It's the 80's and you can still
> count the computers in the world. :)

> Oops... And that was not quite what Mr Cerf meant to do.

I finally got around to finding a reply Vint Cerf wrote to a thread I 
started a year or two ago. The url is 
http://mailman.nanog.org/pipermail/nanog/2010-April/020488.html and 
quoted below in full for future prosperity.

This gives a great behind the scenes view and a clear idea on the 
thought processes involved and why things evolved the way they did. 
Interestingly ipv6 is 128 bits, and I personally would have loved to see 
variable length address structures being implemented, alas. Maybe when 
ipv6 is in need of replacement...

* Begin quote *


Vint Cerf kindly sent through some more explanation.


Begin forwarded message:

Date: Sat, 3 Apr 2010 08:17:28 -0400
From: Vint Cerf <vint at google.com>
To: Mark Smith
<nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org> Cc: Andrew
Gray <3356 at blargh.com>, NANOG List <nanog at nanog.org> Subject: Re:
legacy /8

When the Internet design work began, there were only a few fairly large
networks around. ARPANET was one. The Packet Radio and Packet Satellite
networks were still largely nascent. Ethernet had been implemented in
one place: Xerox PARC. We had no way to know whether the Internet idea
was going to work. We knew that the NCP protocol was inadequate for
lossy network operation (think: PRNET and Ethernet in particular). This
was a RESEARCH project. We assumed that national scale networks were
expensive so there would not be too many of them.  And we certainly did
not think there would be many built for a proof of concept. So 8 bits
seemed reasonable. Later, with local networks becoming popular, we
shifted to the class A-D address structure and when class B was near
exhaustion, the NSFNET team (I think specifically Hans-Werner Braun but
perhaps others also) came up with CIDR and the use of masks to indicate
the size of the "network" part of the 32 bit address structure. By 1990
(7 years after the operational start of the Internet and 17 years since
its basic design), it seemed clear that the 32 bit space would be
exhausted and the long debate about IPng that became IPv6 began. CIDR
slowed the rate of consumption through more efficient allocation of
network addresses but now, in 2010, we face imminent exhaustion of the
32 bit structure and must move to IPv6.

Part of the reason for not changing to a larger address space sooner
had to do with the fact that there were a fairly large number of
operating systems in use and every one of them would have had to be
modified to run a new TCP and IP protocol. So the "hacks" seemed the
more convenient alternative. There had been debates during the 1976
year about address size and proposals ranged from 32 to 128 bit to
variable length address structures. No convergence appeared and, as the
program manager at DARPA, I felt it necessary to simply declare a
choice. At the time (1977), it seemed to me wasteful to select 128 bits
and variable length address structures led to a lot of processing
overhead per packet to find the various fields of the IP packet format.
So I chose 32 bits.


* end quote *

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