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RFC 4632, "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan", August 2006

Source of RFC: grow (ops)

Errata ID: 1577
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT

Reported By: Tony Li
Date Reported: 2008-10-23
Held for Document Update by: Ron Bonica

Section 3.1 says:

   For example, the legacy "Class B" network 172.16.0.0, with an implied
   network mask of 255.255.0.0, is defined as the prefix 172.16.0.0/16,
   the "/16" indicating that the mask to extract the network portion of
   the prefix is a 32-bit value where the most significant 16 bits are
   ones and the least significant 16 bits are zeros.  Similarly, the
   legacy "Class C" network number 192.168.99.0 is defined as the prefix
   192.168.99.0/24; the most significant 24 bits are ones and the least
   significant 8 bits are zeros.


It should say:

   For example, the legacy "Class B" network 172.16.0.0, with an implied
   network mask of 255.255.0.0, is defined as the prefix 172.16.0.0/16,
   the "/16" indicating that the mask to extract the network portion of
   the prefix is a 32-bit value where the most significant 16 bits are
   ones and the least significant 16 bits are zeros.  Similarly, the
   legacy "Class C" network number 192.168.99.0 is defined as the prefix
   192.168.99.0/24; the most significant 24 bits are ones and the least
   significant 8 bits are zeros.

   In cases where a prefix has 1, 2, or 3 trailing insignificant
   octets, it is permissible to elide the insignificant octets and
   trailing '.' separators. Thus, 172.16.0.0/16 may also be represented
   as 172.16/16, and 192.168.99.0/24 is equivalent to 192.168.99/24.



Notes:

This adds some clarifying text and documents a common convention for displaying prefixes. It was never the intention of the authors to exclude the alternative notation and it has since come into vogue. It should be explicitly documented as being acceptable.

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