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"The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets:

 10.0.0.0     -   10.255.255.255  (CIDR block: 10.0.0.0/8)
 172.16.0.0   -   172.31.255.255  (CIDR block: 172.16.0.0/12)
 192.168.0.0  -   192.168.255.255 (CIDR block: 192.168.0.0/16)

I have two questions:

  1. why the CIDR prefix is /8, /12, /16 respectively? why the CIDR prefix is not /8, /16, /24 which matches the prefixs of Class A, Class B and Class C respectively?

  2. since Network address classes are dead and we are in CIDR world, so we can use any prefix we want such as /9, /17, /25 etc, so why there are still "default" CIDR prefixs /8, /12, /16 being used?

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  • I think you are really reading too much into it. There was a need to set aside some address ranges for use inside companies that would never be used on the public Internet. The IETF chose some ranges and created an RFC to tell companies (ISPs are companies too) to never use those addresses between companies, and the ISPs have blocks on their edges to drop packets using addresses in those ranges. There is nothing special about the Private address ranges, they were simply chosen to be blocked on the public Internet so that companies had something to use internally.
    – Ron Maupin ♦
    Commented Sep 18 at 16:54
  • Read RFC 1918 and it explains all about the Private address ranges.
    – Ron Maupin ♦
    Commented Sep 18 at 16:56
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    I already answered this. ONE CLASS A, 16 CLASS B's, 256 CLASS C's /8, /12, and /16 are the respective sizes to get those.
    – Ricky
    Commented Sep 18 at 20:40

1 Answer 1

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  1. why the CIDR prefix is /8, /12, /16 respectively? why the CIDR prefix is not /8, /16, /24 which matches the prefixs of Class A, Class B and Class C respectively?

You are mixing Classes with Classless Inter-Domain Routing (CIDR). CIDR means there are no classes, so CIDR does not need to respect classes. Forget network address classes, they are obsolete.

  1. since Network address classes are dead and we are in CIDR world, so we can use any prefix we want such as /9, /17, /25 etc, so why there are still "default" CIDR prefixs /8, /12, /16 being used?

The Private address ranges were simply chosen rather arbitrarily. IP, itself, knows nothing about Private or public addresses. The Private address ranges were chosen to try to provide enough space for companies to use with some expansion in mind. The only difference between Private and other addresses is that the ISPs agree not to forward packets with Private addresses. IP treats Private addresses like any other IP addresses.

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