Using /126 for IPv6 router links

Steven Bellovin smb at
Sun Jan 24 22:01:21 UTC 2010

On Jan 24, 2010, at 4:45 PM, Mark Smith wrote:

> Actually, from what Christian Huitema says in his "IPv6: The New
> Internet Protocol" book, the original IPv6 address size was 64 bits,
> derived from Steve Deering's Simple Internet Protocol proposal.
> IIRC, they doubled it to 128 bits to specifically have 64 bits as the
> host portion, to allow for autoconfiguration.

Actually, Scott Bradner and I share most of the credit (or blame) for the change from 64 bits to 128.

During the days of the IPng directorate, quite a number of different alternatives were considered.  At one point, there was a compromise proposal known as the "Big 10" design, because it was propounded at the Big Ten Conference Center near O'Hare.  One feature of it was addresses of length 64, 128, 192, or 256 bits, determined by the high-order two bits.  That deal fell apart for reasons I no longer remember; SIPP was the heir apparent at that point.  Scott and I pushed back, saying that 64 bits was too few to allow for both growth and for innovative uses of the address.  We offered 128 bits as a compromise; it was accepted, albeit grudgingly.  The stateless autoconfig design came later.

		--Steve Bellovin,

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