RFC 4291, "IP Version 6 Addressing Architecture", February 2006

Note: This RFC has been updated by RFC 5952, RFC 6052, RFC 7136, RFC 7346, RFC 7371, RFC 8064

Errata ID: 6596
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Martin D Kealey
Date Reported: 2021-06-03

Section 2.2 says:

   2. Due to some methods of allocating certain styles of IPv6
      addresses, it will be common for addresses to contain long strings
      of zero bits.  In order to make writing addresses containing zero
      bits easier, a special syntax is available to compress the zeros.
      The use of "::" indicates one or more groups of 16 bits of zeros.
      The "::" can only appear once in an address.  The "::" can also be
      used to compress leading or trailing zeros in an address.

It should say:

EITHER

   2. Due to some methods of allocating certain styles of IPv6
      addresses, it will be common for addresses to contain long strings
      of zero bits.  In order to make writing addresses containing zero
      bits easier, a special syntax is available to compress the zeros.
      The use of "::" indicates one or more "0" pieces, or two or more when
      leading or trailing. The "::" can only appear once in an address.

OR

   2. Due to some methods of allocating certain styles of IPv6
      addresses, it will be common for addresses to contain long strings
      of zero bits.  In order to make writing addresses containing zero
      bits easier, a special syntax is available to compress the zeros.
      The use of "::" indicates one or more pieces of 16 bits of zeros,
      or two or more when leading or trailing. The "::" can only appear
      once in an address.

Notes:

1. The existing wording would permit "::" to be used in leading or trailing position to compress a single 16-bit piece, leading to the conclusion that "::0:0:0:0:0:0:1" is a valid way to write the loopback address, with eight colons.

Many consumers of textual IPv6 addresses would reject this out of hand as "too many colons", and it seems questionable that such an interpretation was ever intended. Enforcing this on producers would improve interoperability.

2. The preceding paragraphs defines "piece", but the undefined term "group" is used in this paragraph, so by changing that term I hope to clarify that this is a textual substitution, necessarily aligned to a multiple of 16 bits.

I am uncomfortable with the phrasing "pieces of 16 bits of zeros", but I offer it as a single-word change to effect my point (2); I leave it to the discretion of the editor which of my alternatives to adopt.

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