Nerd question on private network addressing...

[sirwilliam]

Question: Why did the IETF specifically direct the IANA to reserve the specific ranges for IPv4 private network addresses?

Question: Why the specific ranges?

(10.0.0.0 – 10.255.255.255, 172.16.0.0 – 172.31.255.255, and 192.168.0.0 – 192.168.255.255)


Those questions kind of go hand and hand but it peaked my curiosity today so please help!

[complacent]

i know they wanted to be able to have private space in each original class before cidr was around.

beyond that, i'm not sure.

[Sabre]

I'm not sure if this is 100% correct, but is seems plausible:

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.)

[sirwilliam]

Makes sense. THanks.

[scheherazade]

Not sure if this is the response you are looking for, but here's my understanding of the practicalities :



If the addresses were allowed to exist 'in the wild', then there could be a machine out there with those addresses.

So if your router tries to assign a computer on your LAN 192.168.0.122, and that address already exists on the internet, then who should respond if you do ping 192.168.0.122? The computer on your lan? The computer on the internet? Both?

Furthermore, if there was not a dedicated [non conflicting] range, then routers would not be able to follow a simple scheme :
192.168.0.100
192.168.0.101
192.168.0.102
192.168.0.103
...

They would have to look for available public addresses, and give you private addresses that aren't used in public...

But then that defeats the whole purpose of private vs public, if public can 'bump off' private addresses by simply assigning that address in public (making it unavailable for private use).

Someone had to decide what range would never be used publicly to avoid conflicts, and that's what they chose.

-scheherazade