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Internet Engineering Task Force (IETF)                       S. KawamuraRequest for Comments: 5952                             NEC BIGLOBE, Ltd.Updates:4291                                               M. KawashimaCategory: Standards Track                       NEC AccessTechnica, Ltd.ISSN: 2070-1721                                              August 2010A Recommendation for IPv6 Address Text RepresentationAbstract   As IPv6 deployment increases, there will be a dramatic increase in   the need to use IPv6 addresses in text.  While the IPv6 address   architecture inSection 2.2 of RFC 4291 describes a flexible model   for text representation of an IPv6 address, this flexibility has been   causing problems for operators, system engineers, and users.  This   document defines a canonical textual representation format.  It does   not define a format for internal storage, such as within an   application or database.  It is expected that the canonical format   will be followed by humans and systems when representing IPv6   addresses as text, but all implementations must accept and be able to   handle any legitimateRFC 4291 format.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5952.Kawamura & Kawashima         Standards Track                    [Page 1]

RFC 5952                IPv6 Text Representation             August 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Kawamura & Kawashima         Standards Track                    [Page 2]

RFC 5952                IPv6 Text Representation             August 2010Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .41.1.  Requirements Language  . . . . . . . . . . . . . . . . . .42.  Text Representation Flexibility ofRFC 4291  . . . . . . . . .42.1.  Leading Zeros in a 16-Bit Field  . . . . . . . . . . . . .42.2.  Zero Compression . . . . . . . . . . . . . . . . . . . . .52.3.  Uppercase or Lowercase . . . . . . . . . . . . . . . . . .63.  Problems Encountered with the Flexible Model . . . . . . . . .63.1.  Searching  . . . . . . . . . . . . . . . . . . . . . . . .63.1.1.  General Summary  . . . . . . . . . . . . . . . . . . .63.1.2.  Searching Spreadsheets and Text Files  . . . . . . . .63.1.3.  Searching with Whois . . . . . . . . . . . . . . . . .63.1.4.  Searching for an Address in a Network Diagram  . . . .73.2.  Parsing and Modifying  . . . . . . . . . . . . . . . . . .73.2.1.  General Summary  . . . . . . . . . . . . . . . . . . .73.2.2.  Logging  . . . . . . . . . . . . . . . . . . . . . . .73.2.3.  Auditing: Case 1 . . . . . . . . . . . . . . . . . . .83.2.4.  Auditing: Case 2 . . . . . . . . . . . . . . . . . . .83.2.5.  Verification . . . . . . . . . . . . . . . . . . . . .83.2.6.  Unexpected Modifying . . . . . . . . . . . . . . . . .83.3.  Operating  . . . . . . . . . . . . . . . . . . . . . . . .83.3.1.  General Summary  . . . . . . . . . . . . . . . . . . .83.3.2.  Customer Calls . . . . . . . . . . . . . . . . . . . .93.3.3.  Abuse  . . . . . . . . . . . . . . . . . . . . . . . .93.4.  Other Minor Problems . . . . . . . . . . . . . . . . . . .93.4.1.  Changing Platforms . . . . . . . . . . . . . . . . . .93.4.2.  Preference in Documentation  . . . . . . . . . . . . .93.4.3.  Legibility . . . . . . . . . . . . . . . . . . . . . .94.  A Recommendation for IPv6 Text Representation  . . . . . . . .104.1.  Handling Leading Zeros in a 16-Bit Field . . . . . . . . .104.2.  "::" Usage . . . . . . . . . . . . . . . . . . . . . . . .104.2.1.  Shorten as Much as Possible  . . . . . . . . . . . . .104.2.2.  Handling One 16-Bit 0 Field  . . . . . . . . . . . . .104.2.3.  Choice in Placement of "::"  . . . . . . . . . . . . .104.3.  Lowercase  . . . . . . . . . . . . . . . . . . . . . . . .105.  Text Representation of Special Addresses . . . . . . . . . . .116.  Notes on Combining IPv6 Addresses with Port Numbers  . . . . .117.  Prefix Representation  . . . . . . . . . . . . . . . . . . . .128.  Security Considerations  . . . . . . . . . . . . . . . . . . .129.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .1210. References . . . . . . . . . . . . . . . . . . . . . . . . . .1210.1. Normative References . . . . . . . . . . . . . . . . . . .1210.2. Informative References . . . . . . . . . . . . . . . . . .13Appendix A.  For Developers  . . . . . . . . . . . . . . . . . . .14Kawamura & Kawashima         Standards Track                    [Page 3]

RFC 5952                IPv6 Text Representation             August 20101.  Introduction   A single IPv6 address can be text represented in many ways.  Examples   are shown below.      2001:db8:0:0:1:0:0:1      2001:0db8:0:0:1:0:0:1      2001:db8::1:0:0:1      2001:db8::0:1:0:0:1      2001:0db8::1:0:0:1      2001:db8:0:0:1::1      2001:db8:0000:0:1::1      2001:DB8:0:0:1::1   All of the above examples represent the same IPv6 address.  This   flexibility has caused many problems for operators, systems   engineers, and customers.  The problems are noted inSection 3.  A   canonical representation format to avoid problems is introduced inSection 4.1.1.  Requirements Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].2.  Text Representation Flexibility ofRFC 4291   Examples of flexibility inSection 2.2 of [RFC4291] are described   below.2.1.  Leading Zeros in a 16-Bit Field      'It is not necessary to write the leading zeros in an individual      field.'   Conversely, it is also not necessary to omit leading zeros.  This   means that it is possible to select from representations such as   those in the following example.  The final 16-bit field is different,   but all of these addresses represent the same address.Kawamura & Kawashima         Standards Track                    [Page 4]

RFC 5952                IPv6 Text Representation             August 2010      2001:db8:aaaa:bbbb:cccc:dddd:eeee:0001      2001:db8:aaaa:bbbb:cccc:dddd:eeee:001      2001:db8:aaaa:bbbb:cccc:dddd:eeee:01      2001:db8:aaaa:bbbb:cccc:dddd:eeee:12.2.  Zero Compression      'A special syntax is available to compress the zeros.  The use of      "::" indicates one or more groups of 16 bits of zeros.'   It is possible to select whether or not to omit just one 16-bit 0   field.      2001:db8:aaaa:bbbb:cccc:dddd::1      2001:db8:aaaa:bbbb:cccc:dddd:0:1   In cases where there is more than one field of only zeros, there is a   choice of how many fields can be shortened.      2001:db8:0:0:0::1      2001:db8:0:0::1      2001:db8:0::1      2001:db8::1   In addition,Section 2.2 of [RFC4291] notes,      'The "::" can only appear once in an address.'   This gives a choice on where in a single address to compress the   zero.      2001:db8::aaaa:0:0:1      2001:db8:0:0:aaaa::1Kawamura & Kawashima         Standards Track                    [Page 5]

RFC 5952                IPv6 Text Representation             August 20102.3.  Uppercase or Lowercase   [RFC4291] does not mention any preference of uppercase or lowercase.      2001:db8:aaaa:bbbb:cccc:dddd:eeee:aaaa      2001:db8:aaaa:bbbb:cccc:dddd:eeee:AAAA      2001:db8:aaaa:bbbb:cccc:dddd:eeee:AaAa3.  Problems Encountered with the Flexible Model3.1.  Searching3.1.1.  General Summary   A search of an IPv6 address if conducted through a UNIX system is   usually case sensitive and extended options that allow for regular   expression use will come in handy.  However, there are many   applications in the Internet today that do not provide this   capability.  When searching for an IPv6 address in such systems, the   system engineer will have to try each and every possibility to search   for an address.  This has critical impacts, especially when trying to   deploy IPv6 over an enterprise network.3.1.2.  Searching Spreadsheets and Text Files   Spreadsheet applications and text editors on GUI systems rarely have   the ability to search for text using regular expression.  Moreover,   there are many non-engineers (who are not aware of case sensitivity   and regular expression use) that use these applications to manage IP   addresses.  This has worked quite well with IPv4 since text   representation in IPv4 has very little flexibility.  There is no   incentive to encourage these non-engineers to change their tool or   learn regular expression when they decide to go dual-stack.  If the   entry in the spreadsheet reads, 2001:db8::1:0:0:1, but the search was   conducted as 2001:db8:0:0:1::1, this will show a result of no match.   One example where this will cause a problem is, when the search is   being conducted to assign a new address from a pool, and a check is   being done to see if it is not in use.  This may cause problems for   the end-hosts or end-users.  This type of address management is very   often seen in enterprise networks and ISPs.3.1.3.  Searching with Whois   The "whois" utility is used by a wide range of people today.  When a   record is set to a database, one will likely check the output to see   if the entry is correct.  If an entity was recorded as 2001:db8::/48,Kawamura & Kawashima         Standards Track                    [Page 6]

RFC 5952                IPv6 Text Representation             August 2010   but the whois output showed 2001:0db8:0000::/48, most non-engineers   would think that their input was wrong and will likely retry several   times or make a frustrated call to the database hostmaster.  If there   was a need to register the same prefix on different systems, and each   system showed a different text representation, this would confuse   people even more.  Although this document focuses on addresses rather   than prefixes, it is worth mentioning the prefix problems because the   problems encountered with addresses and prefixes are mostly equal.3.1.4.  Searching for an Address in a Network Diagram   Network diagrams and blueprints often show what IP addresses are   assigned to a system devices.  In times of trouble shooting there may   be a need to search through a diagram to find the point of failure   (for example, if a traceroute stopped at 2001:db8::1, one would   search the diagram for that address).  This is a technique quite   often in use in enterprise networks and managed services.  Again, the   different flavors of text representation will result in a time-   consuming search leading to longer mean times to restoration (MTTR)   in times of trouble.3.2.  Parsing and Modifying3.2.1.  General Summary   With all the possible methods of text representation, each   application must include a module, object, link, etc. to a function   that will parse IPv6 addresses in a manner such that no matter how it   is represented, they will mean the same address.  Many system   engineers who integrate complex computer systems for corporate   customers will have difficulties finding that their favorite tool   will not have this function, or will encounter difficulties such as   having to rewrite their macros or scripts for their customers.3.2.2.  Logging   If an application were to output a log summary that represented the   address in full (such as 2001:0db8:0000:0000:1111:2222:3333:4444),   the output would be highly unreadable compared to the IPv4 output.   The address would have to be parsed and reformed to make it useful   for human reading.  Sometimes logging for critical systems is done by   mirroring the same traffic to two different systems.  Care must be   taken so that no matter what the log output is, the logs should be   parsed so they are equivalent.Kawamura & Kawashima         Standards Track                    [Page 7]

RFC 5952                IPv6 Text Representation             August 20103.2.3.  Auditing: Case 1   When a router or any other network appliance machine configuration is   audited, there are many methods to compare the configuration   information of a node.  Sometimes auditing will be done by just   comparing the changes made each day.  In this case, if configuration   was done such that 2001:db8::1 was changed to 2001:0db8:0000:0000:   0000:0000:0000:0001 just because the new engineer on the block felt   it was better, a simple diff will show that a different address was   configured.  If this was done on a wide scale network, people will be   focusing on 'why the extra zeros were put in' instead of doing any   real auditing.  Lots of tools are just plain diffs that do not take   into account address representation rules.3.2.4.  Auditing: Case 2   Node configurations will be matched against an information system   that manages IP addresses.  If output notation is different, there   will need to be a script that is implemented to cover for this.  The   result of an SNMP GET operation, converted to text and compared to a   textual address written by a human is highly unlikely to match on the   first try.3.2.5.  Verification   Some protocols require certain data fields to be verified.  One   example of this is X.509 certificates.  If an IPv6 address field in a   certificate was incorrectly verified by converting it to text and   making a simple textual comparison to some other address, the   certificate may be mistakenly shown as being invalid due to a   difference in text representation methods.3.2.6.  Unexpected Modifying   Sometimes, a system will take an address and modify it as a   convenience.  For example, a system may take an input of   2001:0db8:0::1 and make the output 2001:db8::1.  If the zeros were   input for a reason, the outcome may be somewhat unexpected.3.3.  Operating3.3.1.  General Summary   When an operator sets an IPv6 address of a system as 2001:db8:0:0:1:   0:0:1, the system may take the address and show the configuration   result as 2001:DB8::1:0:0:1.  Someone familiar with IPv6 address   representation will know that the right address is set, but not   everyone may understand this.Kawamura & Kawashima         Standards Track                    [Page 8]

RFC 5952                IPv6 Text Representation             August 20103.3.2.  Customer Calls   When a customer calls to inquire about a suspected outage, IPv6   address representation should be handled with care.  Not all   customers are engineers, nor do they have a similar skill level in   IPv6 technology.  The network operations center will have to take   extra steps to humanly parse the address to avoid having to explain   to the customers that 2001:db8:0:1::1 is the same as   2001:db8::1:0:0:0:1.  This is one thing that will never happen in   IPv4 because IPv4 addresses cannot be abbreviated.3.3.3.  Abuse   Network abuse reports generally include the abusing IP address.  This   'reporting' could take any shape or form of the flexible model.  A   team that handles network abuse must be able to tell the difference   between a 2001:db8::1:0:1 and 2001:db8:1::0:1.  Mistakes in the   placement of the "::" will result in a critical situation.  A system   that handles these incidents should be able to handle any type of   input and parse it in a correct manner.  Also, incidents are reported   over the phone.  It is unnecessary to report if the letter is   uppercase or lowercase.  However, when a letter is spelled uppercase,   people tend to specify that it is uppercase, which is unnecessary   information.3.4.  Other Minor Problems3.4.1.  Changing Platforms   When an engineer decides to change the platform of a running service,   the same code may not work as expected due to the difference in IPv6   address text representation.  Usually, a change in a platform (e.g.,   Unix to Windows, Cisco to Juniper) will result in a major change of   code anyway, but flexibility in address representation will increase   the work load.3.4.2.  Preference in Documentation   A document that is edited by more than one author may become harder   to read.3.4.3.  Legibility   Capital case D and 0 can be quite often misread.  Capital B and 8 can   also be misread.Kawamura & Kawashima         Standards Track                    [Page 9]

RFC 5952                IPv6 Text Representation             August 20104.  A Recommendation for IPv6 Text Representation   A recommendation for a canonical text representation format of IPv6   addresses is presented in this section.  The recommendation in this   document is one that complies fully with [RFC4291], is implemented by   various operating systems, and is human friendly.  The recommendation   in this section SHOULD be followed by systems when generating an   address to be represented as text, but all implementations MUST   accept and be able to handle any legitimate [RFC4291] format.  It is   advised that humans also follow these recommendations when spelling   an address.4.1.  Handling Leading Zeros in a 16-Bit Field   Leading zeros MUST be suppressed.  For example, 2001:0db8::0001 is   not acceptable and must be represented as 2001:db8::1.  A single 16-   bit 0000 field MUST be represented as 0.4.2.  "::" Usage4.2.1.  Shorten as Much as Possible   The use of the symbol "::" MUST be used to its maximum capability.   For example, 2001:db8:0:0:0:0:2:1 must be shortened to 2001:db8::2:1.   Likewise, 2001:db8::0:1 is not acceptable, because the symbol "::"   could have been used to produce a shorter representation 2001:db8::1.4.2.2.  Handling One 16-Bit 0 Field   The symbol "::" MUST NOT be used to shorten just one 16-bit 0 field.   For example, the representation 2001:db8:0:1:1:1:1:1 is correct, but   2001:db8::1:1:1:1:1 is not correct.4.2.3.  Choice in Placement of "::"   When there is an alternative choice in the placement of a "::", the   longest run of consecutive 16-bit 0 fields MUST be shortened (i.e.,   the sequence with three consecutive zero fields is shortened in 2001:   0:0:1:0:0:0:1).  When the length of the consecutive 16-bit 0 fields   are equal (i.e., 2001:db8:0:0:1:0:0:1), the first sequence of zero   bits MUST be shortened.  For example, 2001:db8::1:0:0:1 is correct   representation.4.3.  Lowercase   The characters "a", "b", "c", "d", "e", and "f" in an IPv6 address   MUST be represented in lowercase.Kawamura & Kawashima         Standards Track                   [Page 10]

RFC 5952                IPv6 Text Representation             August 20105.  Text Representation of Special Addresses   Addresses such as IPv4-Mapped IPv6 addresses, ISATAP [RFC5214], and   IPv4-translatable addresses [ADDR-FORMAT] have IPv4 addresses   embedded in the low-order 32 bits of the address.  These addresses   have a special representation that may mix hexadecimal and dot   decimal notations.  The decimal notation may be used only for the   last 32 bits of the address.  For these addresses, mixed notation is   RECOMMENDED if the following condition is met: the address can be   distinguished as having IPv4 addresses embedded in the lower 32 bits   solely from the address field through the use of a well-known prefix.   Such prefixes are defined in [RFC4291] and [RFC2765] at the time of   this writing.  If it is known by some external method that a given   prefix is used to embed IPv4, it MAY be represented as mixed   notation.  Tools that provide options to specify prefixes that are   (or are not) to be represented as mixed notation may be useful.   There is a trade-off here where a recommendation to achieve an exact   match in a search (no dot decimals whatsoever) and a recommendation   to help the readability of an address (dot decimal whenever possible)   does not result in the same solution.  The above recommendation is   aimed at fixing the representation as much as possible while leaving   the opportunity for future well-known prefixes to be represented in a   human-friendly manner as tools adjust to newly assigned prefixes.   The text representation method noted inSection 4 should be applied   for the leading hexadecimal part (i.e., ::ffff:192.0.2.1 instead of   0:0:0:0:0:ffff:192.0.2.1).6.  Notes on Combining IPv6 Addresses with Port Numbers   There are many different ways to combine IPv6 addresses and port   numbers that are represented in text.  Examples are shown below.   o  [2001:db8::1]:80   o  2001:db8::1:80   o  2001:db8::1.80   o  2001:db8::1 port 80   o  2001:db8::1p80   o  2001:db8::1#80   The situation is not much different in IPv4, but the most ambiguous   case with IPv6 is the second bullet.  This is due to the "::"usage inKawamura & Kawashima         Standards Track                   [Page 11]

RFC 5952                IPv6 Text Representation             August 2010   IPv6 addresses.  This style is NOT RECOMMENDED because of its   ambiguity.  The [] style as expressed in [RFC3986] SHOULD be   employed, and is the default unless otherwise specified.  Other   styles are acceptable when there is exactly one style for the given   context and cross-platform portability does not become an issue.  For   URIs containing IPv6 address literals, [RFC3986] MUST be followed, as   well as the rules defined in this document.7.  Prefix Representation   Problems with prefixes are the same as problems encountered with   addresses.  The text representation method of IPv6 prefixes should be   no different from that of IPv6 addresses.8.  Security Considerations   This document notes some examples where IPv6 addresses are compared   in text format.  The example onSection 3.2.5 is one that may cause a   security risk if used for access control.  The common practice of   comparing X.509 data is done in binary format.9.  Acknowledgements   The authors would like to thank Jan Zorz, Randy Bush, Yuichi Minami,   and Toshimitsu Matsuura for their generous and helpful comments in   kick starting this document.  We also would like to thank Brian   Carpenter, Akira Kato, Juergen Schoenwaelder, Antonio Querubin, Dave   Thaler, Brian Haley, Suresh Krishnan, Jerry Huang, Roman Donchenko,   Heikki Vatiainen, Dan Wing, and Doug Barton for their input.  Also, a   very special thanks to Ron Bonica, Fred Baker, Brian Haberman, Robert   Hinden, Jari Arkko, and Kurt Lindqvist for their support in bringing   this document to light in IETF working groups.10.  References10.1.  Normative References   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate                  Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2765]      Nordmark, E., "Stateless IP/ICMP Translation Algorithm                  (SIIT)",RFC 2765, February 2000.   [RFC3986]      Berners-Lee, T., Fielding, R., and L. Masinter,                  "Uniform Resource Identifier (URI): Generic Syntax",                  STD 66,RFC 3986, January 2005.Kawamura & Kawashima         Standards Track                   [Page 12]

RFC 5952                IPv6 Text Representation             August 2010   [RFC4291]      Hinden, R. and S. Deering, "IP Version 6 Addressing                  Architecture",RFC 4291, February 2006.10.2.  Informative References   [ADDR-FORMAT]  Bao, C.,"IPv6 Addressing of IPv4/IPv6 Translators",                  Work in Progress, July 2010.   [RFC4038]      Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E.                  Castro, "Application Aspects of IPv6 Transition",RFC 4038, March 2005.   [RFC5214]      Templin, F., Gleeson, T., and D. Thaler, "Intra-Site                  Automatic Tunnel Addressing Protocol (ISATAP)",RFC 5214, March 2008.Kawamura & Kawashima         Standards Track                   [Page 13]

RFC 5952                IPv6 Text Representation             August 2010Appendix A.  For Developers   We recommend that developers use display routines that conform to   these rules.  For example, the usage of getnameinfo() with flags   argument NI_NUMERICHOST in FreeBSD 7.0 will give a conforming output,   except for the special addresses notes inSection 5.  The function   inet_ntop() of FreeBSD7.0 is a good C code reference, but should not   be called directly.  See [RFC4038] for details.Authors' Addresses   Seiichi Kawamura   NEC BIGLOBE, Ltd.   14-22, Shibaura 4-chome   Minatoku, Tokyo  108-8558   JAPAN   Phone: +81 3 3798 6085   EMail: kawamucho@mesh.ad.jp   Masanobu Kawashima   NEC AccessTechnica, Ltd.   800, Shimomata   Kakegawa-shi, Shizuoka  436-8501   JAPAN   Phone: +81 537 23 9655   EMail: kawashimam@necat.nec.co.jpKawamura & Kawashima         Standards Track                   [Page 14]