Daniel Karrenberg, co-author of RFC1918, said this 2017-10-06 on the NANOG mailing list:

  > On 05/10/2017 07:40, Jay R. Ashworth wrote:
  > > Does anyone have a pointer to an *authoritative* source on why
  > >
  > > 10/8
  > > 172.16/12 and
  > > 192.168/16
  > >
  > > were the ranges chosen to enshrine in the RFC? ...
  > 
  > The RFC explains the reason why we chose three ranges from "Class A,B &
  > C" respectively: CIDR had been specified but had not been widely
  > implemented. There was a significant amount of equipment out there that
  > still was "classful".
  > 
  > As far as I recall the choice of the particular ranges were as follows:
  > 
  > 10/8: the ARPANET had just been turned off. One of us suggested it and
  > Jon considered this a good re-use of this "historical" address block. We
  > also suspected that "net 10" might have been hard coded in some places,
  > so re-using it for private address space rather than in inter-AS routing
  > might have the slight advantage of keeping such silliness local.
  > 
  > 172.16/12: the lowest unallocated /12 in class B space.
  > 
  > 192.168/16: the lowest unallocated /16 in class C block 192/8.
  > 
  > In summary: IANA allocated this space just as it would have for any
  > other purpose. As the IANA, Jon was very consistent unless there was a
  > really good reason to be creative.
  > 
  > Daniel (co-author of RFC1918)

And said the same on SuperUser the day after.

* https://superuser.com/a/1257080/38062

And I suppose 127/8 because it's the highest /7 or highest /8 without the MSB on?

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