RFC 1918 network range choices

[feldman at twincreeks.net (Steve Feldman)]

> On Oct 5, 2017, at 4:14 PM, William Herrin <bill at herrin.us> wrote:
> 
> On Thu, Oct 5, 2017 at 1:32 PM, Jerry Cloe <jerry at jtcloe.net> wrote:
> 
>> Several years ago I remember seeing a mathematical justification for it,
>> and I remember thinking at the time it made a lot of sense, but now I can't
>> find it.
>> 
> 
> Hi Jerry,
> 
> If there's special ASIC-friendly math here, beyond what was later
> generalized with CIDR, it's not obvious.
> 
> 10.0:    0000 1010 0000 0000
> 
> 172.16:  1010 1100 0001 0000
> 
> 172.31:  1010 1100 0001 1111
> 
> 192.168: 1100 0000 1010 1000
> 
> AFAIK, it was simply one range each from classes A, B and C.

As mentioned in one of the links posted earlier, 10.0.0.0/8 was the original ARPANET class A assignment.  (See RFC 970, which brings back a lot of memories.)  Once the ARPANET was shut down in 1990 that block was no longer used, so it became available for reuse in RFC1918.

I have a vague recollection of parts of 192.168.0.0/16 being used as default addresses on early Sun systems.  If that's actually true, it might explain that choice.
    Steve