RFC 1918 network range choices

Daniel Karrenberg dfk at ripe.net
Fri Oct 6 18:34:30 UTC 2017

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)

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