Hi Team,
I know the concept of public IP, private IP ranges and its purpose.
But a guy asked me couple of days back why particular range is choosen as private over other.
I think its a Randon pick as they wanted to give private range for users(companies) from each class of network.
10.0.0.0/24
172.16.0.0/20
192.168.0.0/16
Is there any specific reason behind assigning these ranges as private?
Thank you
Sanjeev

Hi Sanjeev,

10.0.0.0/8 was the old ARPANET, which they picked up on 01-Jan-1983. When they shut down the ARPANET in 1990, the 10.0.0.0/8 block was freed. There was much argument about if there should ever be private IP spaces, given that a goal of IPv4 was universal to all hosts on the net.

In then end, practicality won out, and RFC 1597 reserved the now well known private address spaces. When ARPANET went away, the 10.0.0.0/8 allocation was marked as reserved and since it was known that the ARPANET was truly gone (the hosts being moved to MILNET, NSFNET or the Internet) it was decided that this was the best Class A block to allocate.

Note Class A. This was before CIDR. So, the Class A, B and C private address netblocks needed to come out of the correct IP ranges.

I know that 172.16.0.0/12 was picked because it offered the most continuous block of Class B (/16) addresses in the IP space that was in a reserved block. 192.0.0.0/24 was always reserved for the same reason that 0.0.0.0/8 and 128.0.0.0/16 were reserved (first blocks of the old Class C, A and B network blocks) so assigning 192.168.0.0/24 out as private fit well -- 192.0.2.0/24 was already TEST-NET, where you could use them in public documentation without fear of someone trying it (see example.com for another example.)

Hope to Help !!

Ganesh.H

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