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What is a nibble boundary?

A nibble is 4 bits. A nibble boundary is a network mask that aligns with a boundary of 4 bits. The size of the IPv6 prefix to be delegated should match a nibble-aligned boundary to keep addressing plans easily readable and understandable. Moreover, since DNS reverse delegations for IPv6 are based on the closest 4-bit boundaries, the use of nibble boundaries simplifies the management of DNS reverse delegations. In an IPv6 prefix, each hexadecimal character represents one nibble, which is 4 bits. Therefore, the prefix length of a delegated prefix should always be a multiple of 4.


Some examples of nibble boundary masks; 48, 44, 40, 36, 32, 28, 24, etc.


Example of a non-nibble aligned prefix: 2001:0db8:0:4000::/50

  • The range of the IPv6 block is;

  • The subnet only runs from 4000 to 7FFF and hence does not use the entire nibble range.
  • The adjacent block is 2001:0db8:0:8000::/50 and its range is;

  • To have a DNS reverse zone delegation covering the /50, in total 4 delegation entries should be created for the nearest nibble(one for each /52).


Example of a nibble-aligned prefix; 2001:0db8::/48

  • The range is;

  • The entire nibble range is used, 0 to f
  • The adjacent block is 2001:0db8:1::/48 and its range is;

  • Only one DNS delegation may be configured to cover the whole range.
on Thursday October 29 by
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