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Network Working Group                                             R. ElzRequest for Comments: 1924                       University of MelbourneCategory: Informational                                     1 April 1996A Compact Representation of IPv6 AddressesStatus 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   bytes.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, 10.1.0.23 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   address.   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 otherElz                          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.   Examples:        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:        1080:0:0:0:8:800:200C:417A   may be represented as        1080::8:800:200C:417A   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   address:        0:0:0:0:0:0:0:0   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   base 85 representation all possible 2^128 IPv6 addresses can clearly   be encoded.   84^20 is 305904398238499908683087849324518834176, clearly not   sufficient, 21 characters would be needed to encode using base 84,   this wastage of notational space cannot be tolerated.   On the other hand, 94^19 is just   30862366077815087592879016454695419904, also insufficient to encode   all 2^128 different IPv6 addresses, so 20 characters would be needed   even with base 94 encoding.  As there are just 94 ASCII characters   (excluding control characters, space, and del) base 94 is the largest   reasonable value that can be used.  Even if space were allowed, base   95 would still require 20 characters.   Thus, any value between 85 and 94 inclusive could reasonably be   chosen.  Selecting 85 allows the use of the smallest possible subset   of the ASCII characters, enabling more characters to be retained for   other uses, eg, to delimit the address.4.2. The Character Set   The character set to encode the 85 base85 digits, is defined to be,   in ascending order:             '0'..'9', 'A'..'Z', 'a'..'z', '!', '#', '$', '%', '&', '(',             ')', '*', '+', '-', ';', '<', '=', '>', '?', '@', '^', '_',             '`', '{', '|', '}', and '~'.   This set has been chosen with considerable care.  From the 94   printable ASCII characters, the following nine were omitted:Elz                          Informational                      [Page 3]

RFC 1924       A Compact Representation of IPv6 Addresses   1 April 1996      '"' and "'", which allow the representation of IPv6 addresses to      be quoted in other environments where some of the characters in      the chosen character set may, unquoted, have other meanings.      ',' to allow lists of IPv6 addresses to conveniently be written,      and '.' to allow an IPv6 address to end a sentence without      requiring it to be quoted.      '/' so IPv6 addresses can be written in standard CIDR      address/length notation, and ':' because that causes problems when      used in mail headers and URLs.      '[' and ']', so those can be used to delimit IPv6 addresses when      represented as text strings, as they often are for IPv4,      And last, '\', because it is often difficult to represent in a way      where it does not appear to be a quote character, including in the      source of this document.5. Converting an IPv6 address to base 85.   The conversion process is a simple one of division, taking the   remainders at each step, and dividing the quotient again, then   reading up the page, as is done for any other base conversion.   For example, consider the address shown above        1080:0:0:0:8:800:200C:417A   In decimal, considered as a 128 bit number, that is   21932261930451111902915077091070067066.   As we divide that successively by 85 the following remainders emerge:   51, 34, 65, 57, 58, 0, 75, 53, 37, 4, 19, 61, 31, 63, 12, 66, 46, 70,   68, 4.   Thus in base85 the address is:        4-68-70-46-66-12-63-31-61-19-4-37-53-75-0-58-57-65-34-51.   Then, when encoded as specified above, this becomes:        4)+k&C#VzJ4br>0wv%Yp   This procedure is trivially reversed to produce the binary form of   the address from textually encoded format.Elz                          Informational                      [Page 4]

RFC 1924       A Compact Representation of IPv6 Addresses   1 April 19966. Additional Benefit   Apart from generally reducing the length of an IPv6 address when   encode in a textual format, this scheme also has the benefit of   returning IPv6 addresses to a fixed length representation, leading   zeroes are never omitted, thus removing the ugly and awkward variable   length representation that has previously been recommended.7. Implementation Issues   Many current processors do not find 128 bit integer arithmetic, as   required for this technique, a trivial operation.  This is not   considered a serious drawback in the representation, but a flaw of   the processor designs.   It may be expected that future processors will address this defect,   quite possibly before any significant IPv6 deployment has been   accomplished.8. Security Considerations   By encoding addresses in this form, it is less likely that a casual   observer will be able to immediately detect the binary form of the   address, and thus will find it harder to make immediate use of the   address.  As IPv6 addresses are not intended to be learned by humans,   one reason for which being that they are expected to alter in   comparatively short timespan, by human perception, the somewhat   challenging nature of the addresses is seen as a feature.   Further, the appearance of the address, as if it may be random   gibberish in a compressed file, makes it much harder to detect by a   packet sniffer programmed to look for bypassing addresses.Elz                          Informational                      [Page 5]

RFC 1924       A Compact Representation of IPv6 Addresses   1 April 19969. References   [IPv6]        Internet Protocol, Version 6 (IPv6) Specification,                 S. Deering, R. Hinden,RFC 1883, January 4, 1996.   [AddrSpec]    IP Version 6 Addressing Architecture,                 R. Hinden, S. Deering,RFC 1884, January 4, 1996.10. Author's Address   Robert Elz   Computer Science   University of Melbourne   Parkville, Victoria, 3052   Australia   EMail: kre@munnari.OZ.AUElz                          Informational                      [Page 6]