A Compact Representation of IPv6 Addresses
RFC 1924

Document Type RFC - Informational (April 1996; No errata)
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Network Working Group                                             R. Elz
Request for Comments: 1924                       University of Melbourne
Category: Informational                                     1 April 1996

               A Compact Representation of IPv6 Addresses

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

1. Abstract

   IPv6 addresses, being 128 bits long, need 32 characters to write in
   the general case, if standard hex representation, is used, plus more
   for any punctuation inserted (typically about another 7 characters,
   or 39 characters total).  This document specifies a more compact
   representation of IPv6 addresses, which permits encoding in a mere 20

2. Introduction

   It is always necessary to be able to write in characters the form of
   an address, though in actual use it is always carried in binary.  For
   IP version 4 (IP Classic) the well known dotted quad format is used.
   That is, is one such address.  Each decimal integer
   represents a one octet of the 4 octet address, and consequently has a
   value between 0 and 255 (inclusive).  The written length of the
   address varies between 7 and 15 bytes.

   For IPv6 however, addresses are 16 octets long [IPv6], if the old
   standard form were to be used, addresses would be anywhere between 31
   and 63 bytes, which is, of course, untenable.

   Because of that, IPv6 had chosen to represent addresses using hex
   digits, and use only half as many punctuation characters, which will
   mean addresses of between 15 and 39 bytes, which is still quite long.
   Further, in an attempt to save more bytes, a special format was
   invented, in which a single run of zero octets can be dropped, the
   two adjacent punctuation characters indicate this has happened, the
   number of missing zeroes can be deduced from the fixed size of the

   In most cases, using genuine IPv6 addresses, one may expect the
   address as written to tend toward the upper limit of 39 octets, as
   long strings of zeroes are likely to be rare, and most of the other

Elz                          Informational                      [Page 1]
RFC 1924       A Compact Representation of IPv6 Addresses   1 April 1996

   groups of 4 hex digits are likely to be longer than a single non-zero
   digit (just as MAC addresses typically have digits spread throughout
   their length).

   This document specifies a new encoding, which can always represent
   any IPv6 address in 20 octets.  While longer than the shortest
   possible representation of an IPv6 address, this is barely longer
   than half the longest representation, and will typically be shorter
   than the representation of most IPv6 addresses.

3. Current formats

   [AddrSpec] specifies that the preferred text representation of IPv6
   addresses is in one of three conventional forms.

   The preferred form is x:x:x:x:x:x:x:x, where the 'x's are the
   hexadecimal values of the eight 16-bit pieces of the address.


        FEDC:BA98:7654:3210:FEDC:BA98:7654:3210  (39 characters)

        1080:0:0:0:8:800:200C:417A  (25 characters)

   The second, or zero suppressed, form allows "::" to indicate multiple
   groups of suppressed zeroes, hence:


   may be represented as


   a saving of just 5 characters from this typical address form, and
   still leaving 21 characters.

   In other cases the saving is more dramatic, in the extreme case, the


   that is, the unspecified address, can be written as


   This is just 2 characters, which is a considerable saving.  However
   such cases will rarely be encountered.

Elz                          Informational                      [Page 2]
RFC 1924       A Compact Representation of IPv6 Addresses   1 April 1996

   The third possible form mixes the new IPv6 form with the old IPv4
   form, and is intended mostly for transition, when IPv4 addresses are
   embedded into IPv6 addresses.  These can be considerably longer than
   the longest normal IPv6 representation, and will eventually be phased
   out.  Consequently they will not be considered further here.

4. The New Encoding Format

   The new standard way of writing IPv6 addresses is to treat them as a
   128 bit integer, encode that in base 85 notation, then encode that
   using 85 ASCII characters.

4.1. Why 85?

   2^128 is 340282366920938463463374607431768211456.  85^20 is
   387595310845143558731231784820556640625, and thus in 20 digits of
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