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An internationalized domain name (IDN) is an Internet domain name that contains at least one label that is displayed in software applications, in whole or in part, in a language-specific script or alphabet, such as Chinese, Russian or the Latin-based languages with diacritics, such as French. These writing systems are encoded by computers in multi-byte Unicode. Internationalized domain names are stored in the Domain Name System as ASCII strings using Punycode transcription.

The Domain Name System, which performs a lookup service to translate user-friendly names into network addresses for locating Internet resources, is restricted to the use of ASCII characters, a technical limitation that initially set the standard for acceptable domain names. The internationalization of domain names is a technical solution to translate names written in language-native scripts into an ASCII text representation that is compatible with the Domain Name System. Internationalized domain names can only be used with applications that are specifically designed for such use, and they require no changes in the infrastructure of the Internet.

IDN was originally proposed in December 1996 by Martin Dürst and implemented in 1998 by Tan Juay Kwang and Leong Kok Yong under the guidance of T.W. Tan. After much debate and many competing proposals, a system called Internationalizing Domain Names in Applications (IDNA)
was adopted as a standard, and has been implemented in several top-level domains.

In IDNA, the term internationalized domain name means specifically any domain name consisting only of labels to which the IDNA ToASCII algorithm (see below) can be successfully applied. In March 2008, the IETF formed a new IDN working group to update the current IDNA protocol.

In October 2009, the Internet Corporation for Assigned Names and Numbers (ICANN) approved the creation of country code top-level domains (ccTLDs) in the Internet that use the IDNA standard for native language scripts.

Internationalizing Domain Names in Applications

Internationalizing Domain Names in Applications (IDNA) is a mechanism defined in 2003 for handling internationalized domain names containing non-ASCII characters.While much of the Domain Name System can technically support non-ASCII characters, applications such as e-mail and web browsers restrict domain names to what can be used as a hostname. Rather than redesigning the existing DNS infrastructure, it was decided that non-ASCII domain names should be converted to a suitable ASCII-based form by web browsers and other user applications; IDNA specifies how this conversion is to be done.

IDNA was designed for maximum backward compatibility with the existing DNS system, which was designed for use with names using only a subset of the ASCII character set.

An IDNA-enabled application is able to convert between the restricted-ASCII and non-ASCII representations of a domain, using the ASCII form in cases in which it is needed (such as for DNS lookup), but being able to present the more readable non-ASCII form to users. Applications that do not support IDNA will not be able to handle domain names with non-ASCII characters, but will still be able to access such domains if given the (usually rather cryptic) ASCII equivalent.

ICANN issued guidelines for the use of IDNA in June 2003, and it was already possible to register .jp domains using this system in July 2003 and .info domains in March 2004. Several other top-level domain registries started accepting registrations in 2004 and 2005. IDN Guidelines were first created in June 2003, and have been updated to respond to phishing concerns in November 2005. An ICANN working group focused on country code domain names at the top level was formed in November 2007 and promoted jointly by the country code supporting organization and the Governmental Advisory Committee.

Mozilla 1.4, Netscape 7.1, Opera 7.11 were among the first applications to support IDNA. A browser plugin is available for Internet Explorer 6 to provide IDN support. Internet Explorer 7.0 and Windows Vista's URL APIs provide native support for IDN.

ToASCII and ToUnicode

The conversions between ASCII and non-ASCII forms of a domain name are accomplished by algorithms called ToASCII and ToUnicode. These algorithms are not applied to the domain name as a whole, but rather to individual labels. For example, if the domain name is, then the labels are www, example, and com. ToASCII or ToUnicode are applied to each of these three separately.

The details of these two algorithms are complex, and are specified in RFC 3490. The following gives an overview of their function.

ToASCII leaves unchanged any ASCII label, but will fail if the label is unsuitable for the Domain Name System.If given a label containing at least one non-ASCII character, ToASCII will apply the Nameprep algorithm, which converts the label to lowercase and performs other normalization, and will then translate the result to ASCII using Punycodebefore prepending the four-character string " ". This four-character string is called the ASCII Compatible Encoding (ACE) prefix, and is used to distinguish Punycode encoded labels from ordinary ASCII labels.The ToASCII algorithm can fail in several ways; for example, the final string could exceed the 63-character limit of a DNS name. A label for which ToASCII fails cannot be used in an internationalized domain name.

The function ToUnicode reverses the action of ToASCII, stripping off the ACE prefix and applying the Punycode decode algorithm. It does not reverse the Nameprep processing, since that is merely a normalization and is by nature irreversible. Unlike ToASCII, ToUnicode always succeeds, because it simply returns the original string if decoding fails. In particular, this means that ToUnicode has no effect on a string that does not begin with the ACE prefix.

Example of IDNA encoding

IDNA encoding may be illustrated using the example domain Bü “Bücher” is German for “books”, and .ch is the ccTLD of Switzerlandmarker. This domain name has two labels, Bücher and ch. The second label is pure ASCII, and is left unchanged. The first label is processed by Nameprep to give bücher, and then converted to Punycode to result in bcher-kva. It is then prepended with to produce bcher-kva. The final domain suitable for use in the DNS is therefore

Top-level domain implementation

The ICANN board approved the establishment of an internationalized top-level domain name working group within the Country Code Names Supporting Organisation (ccNSO) in December 2006. They resolved in June 2007 inter alia to proceed and asked the IDNC Working Group to prepare a proposal, which the group delivered in June 2008, "to recommend mechanisms to introduce a limited number of non-contentious IDN ccTLDs, associated with the ISO 3166-1 two-letter codes in a short time frame to meet near term demand." The group proposed a methodology using ICANN's Fast Track Process based on the ICANN charter to work with the Internet Assigned Numbers Authority (IANA): 1) Identify technical basis of the TLD strings and country code specific processes, select IDN ccTLD personnel and authorities, and prepare documentation; 2) Perform ICANN due diligence process for technical proposal and publish method; 3) Enter delegation process within established IANA procedures.

Starting November 16, 2009, nations and territories may apply for IDN ccTLDs, which may be expected to be operational in mid-2010. Non-Latin alphabet scripts are used by more than half of the world's 1.6 billion Internet users. ICANN expects that Arabic, Chinese, and Russian domains are likely to be the first implementations. ... .مصر‎ .Miṣr Egypt (.eg)


  • 12/1996: Martin Dürst's original Internet Draft proposing UTF5 (the first example of what is known today as an ASCII-compatible encoding (ACE)) – UTF-5 was first defined by Martin Dürst at the University of Zürich in [48931] [48932] [48933]
  • 03/1998: Early Research on IDN at National University of Singapore (NUS), Center for Internet Research (formerly Internet Research and Development Unit – IRDU) led by Prof. Tan Tin Wee (IDN Project team – Lim Juay Kwang and Leong Kok Yong) and subsequently continued under a team at Bioinformatrix Pte. Ltd. (BIX Pte. Ltd.) – an NUS spin-off company led by Prof. S. Subbiah.
  • 07/1998: Geneva INET'98 conference with a BoF discussion on iDNS and APNG General Meeting and Working Group meeting.
  • 07/1998: Asia Pacific Networking Group (APNG, now still in existence [48934] and distinct from a gathering known as APSTAR [48935]) iDNS Working Group formed. [48936]
  • 10/1998: James Seng was recruited to lead further IDN development at BIX Pte. Ltd. by Prof. S. Subbiah.
  • 02/1999: iDNS Testbed launched by BIX Pte. Ltd. under the auspices of APNG with participation from CNNIC, JPNIC, KRNIC, TWNIC, THNIC, HKNIC and SGNIC led by James Seng [48937]
  • 02/1999: Presentation of Report on IDN at Joint APNG-APTLD meeting, at APRICOT'99
  • 03/1999: Endorsement of the IDN Report at APNG General Meeting 1 March 1999.
  • 06/1999: Grant application by APNG jointly with the Centre for Internet Research (CIR), National University of Singapore, to the International Development Research Center (IDRC), a Canadian Government funded international organisation to work on IDN for IPv6. This APNG Project was funded under the Pan Asia R&D Grant administered on behalf of IDRC by the Canadian Committee on Occupational Health and Safety (CCOHS). Principal Investigator: Tan Tin Wee of National University of Singapore. [48938]
  • 07/1999 Tout, Walid R. (WALID Inc.) Filed IDNA patent application number US1999000358043 Method and system for internationalizing domain names. Published 2001-01-30 [48939]
  • 07/1999: [48940]; Renewed 2000 [48941] Internet Draft on UTF5 by James Seng, Martin Dürst and Tan Tin Wee.
  • 08/1999: APTLD and APNG forms a working group to look into IDN issues chaired by Kilnam Chon. [48942]
  • 10/1999: BIX Pte. Ltd. and National University of Singapore together with New York Venture Capital investors, General Atlantic Partners, spun-off the IDN effort into 2 new Singapore companies – International Inc. and Pte. Ltd. that created the first commercial implementation of an IDN Solution for both domain names and IDN email addresses respectively.
  • 11/1999: IETF IDN Birds-of-Feather in Washington was initiated by at the request of IETF officials.
  • 12/1999: InternationalPte. Ltd. launched the first commercial IDN. It was in Taiwanmarker and in Chinese characters under the top-level IDN TLD ".gongsi" (meaning loosely ".com") with endorsement by the Minister of Communications of Taiwan and some major Taiwanese ISPs with reports of over 200 000 names sold in a week in Taiwan, Hong Kong, Singapore, Malaysia, Chinamarker, Australia and USA.
  • Late 1999: Kilnam Chon initiates Task Force on IDNS which led to formation of MINC, the Multilingual Internet Names Consortium. [48943]
  • 01/2000: IETF IDN Working Group formed chaired by James Seng and Marc Blanchet
  • 01/2000: The second ever commercial IDN launch was IDN TLDs in the Tamil Language, corresponding to .com, .net, .org, and .edu. These were launched in India with IT Ministry support by International.
  • 02/2000: Multilingual Internet Names Consortium(MINC) Proposal BoF at IETF Adelaide. [48944]
  • 03/2000: APRICOT 2000 Multilingual DNS session [48945]
  • 04/2000: WALID Inc. (with IDNA patent pending application 6182148) started Registration & Resolving Multilingual Domain Names.
  • 05/2000: Interoperability Testing WG, MINC meeting. San Francisco, chaired by Bill Manning and Y. Yoneya 12 May 2000. [48946]
  • 06/2000: Inaugural Launch of the Multilingual Internet Names Consortium (MINC) in Seoul [48947] to drive the collaborative roll-out of IDN starting from the Asia Pacific. [48948]
  • 07/2000: Joint Engineering TaskForce (JET) initiated in Yokohama to study technical issues led by JPNIC (K.Konishi)
  • 07/2000: Official Formation of CDNC Chinese Domain Name Consortium to resolve issues related to and to deploy Han Character domain names, founded by CNNIC, TWNIC, HKNIC and MONIC in May 2000. [48949] [48950]
  • 03/2001: ICANN Board IDN Working Group formed
  • 07/2001: Japanese Domain Name Association : JDNA Launch Ceremony (July 13, 2001) in Tokyo, Japan.
  • 07/2001: Urdu Internet Names System (July 28, 2001) in Islamabad, Pakistan, Organised Jointly by SDNP and MINC. [48951]
  • 07/2001: Presentation on IDN to the Committee Meeting of the Computer Science and Telecommunications Board, National Academies USA (JULY 11–13, 2001) at University of California School of Information Management and Systems, Berkeley, CA. [48952]
  • 08/2001: MINC presentation and outreach at the Asia Pacific Advanced Network annual conference, Penang, Malaysia 20 August 2001
  • 10/2001: Joint MINC-CDNC Meeting in Beijing 18–20 October 2001
  • 11/2001: ICANN IDN Committee formed
  • 12/2001: Joint ITU-WIPO Symposium on Multilingual Domain Names organised in association with MINC, 6–7 Dec 2001, International Conference Center, Geneva.
  • 01/2003: Free implementation of StringPrep, Punycode, and IDNA release in GNU Libidn.
  • 03/2003: Publication of RFC 3454, RFC 3490, RFC 3491 and RFC 3492
  • 06/2003: Publication of ICANN IDN Guidelines for registries Adopted by .cn, .info, .jp, .org, and .tw registries.
  • 05/2004: Publication of RFC 3743, Joint Engineering Team (JET) Guidelines for Internationalized Domain Names (IDN) Registration and Administration for Chinese, Japanese, and Korean
  • 03/2005: First Study Group 17 of ITU-T meeting on Internationalized Domain Names [48953]
  • 05/2005: .IN ccTLD (India) creates expert IDN Working Group to create solutions for 22 official languages
  • 04/2006: ITU Study Group 17 meeting in Korea gave final approval to the Question on Internationalized Domain Names [48954]
  • 06/2006: Workshop on IDN at ICANN meeting at Marrakech, Morocco
  • 11/2006: ICANN GNSO IDN Working Group created to discuss policy implications of IDN TLDs. Ram Mohan elected Chair of the IDN Working Group.
  • 12/2006: ICANN meeting at São Paulo discusses status of lab tests of IDNs within the root.
  • 01/2007: Tamil and Malayalam variant table work completed by India's C-DAC and Afilias
  • 03/2007: ICANN GNSO IDN Working Group completes work, Ram Mohan presents report at ICANN Lisboa meeting.
  • 10/2007: Eleven IDNA top-level domains were added to the root nameservers in order to evaluate the use of IDNA at the top level of the DNS.
  • 01/2008: ICANN: Successful Evaluations of .test IDN TLDs
  • 04/2008: IETF IDNAbis WG chaired by Vint Cerf continues the work to update IDNA
  • 06/2008: ICANN board votes to develop final fast-track implementation proposal for a limited number of IDN ccTLDS.
  • 10/2008: ICANN Seeks Interest in IDN ccTLD Fast-Track Process
  • 9/2009: ICANN puts IDN ccTLD proposal on agenda for Seoul meeting in October 2009
  • 10/2009: ICANN approves the registration of IDN names in the root of the DNS through the IDN ccTLD Fast-Track process at its meeting in Seoul, Oct. 26–30, 2009.
  • .مصر‎ .Miṣr Egypt (.eg)

Top-level domains known to accept IDN registration

Non-IDNA or non-ICANN registries that support non-ASCII domain names

There are other registries that support non-ASCII domain names. The company in Thailand supports .com registrations via its own modified domain name system, ThaiURL. Because these companies, and other organizations that offer modified DNS systems, do not subject themselves to ICANN's control, they must be regarded as alternate DNS roots. Domains registered with them will therefore not be supported by most Internet service providers, and as a result most users will not be able to look up such domains without manually configuring their computers to use the alternate DNS.

ASCII spoofing concerns

The use of Unicode in domain names makes it potentially easier to spoof web sites visited by World Wide Web users as the visual representation of an IDN string in a web browser may appear identical to another, depending on the font used. For example, Unicode character U+0430, Cyrillic small letter a, can look identical to Unicode character U+0061, Latin small letter a, used in English.

In December 2001 Evgeniy Gabrilovich and Alex Gontmakher, both from the Technionmarker Institute of Technology in Israel, published a paper titled "The Homograph Attack", which described an attack that used Unicode URLs to spoof a website URL. To prove the feasibility of this kind of attack, the researchers successfully registered a variant of the domain name which incorporated Russian language characters.

These kind of problems were anticipated before IDN was introduced, and guidelines were issued to registries to try to avoid or reduce the problem. For example, it was advised that registries only accept characters from the Latin alphabet and that of their own country, not all of Unicode characters, but this advice was neglected by major TLDs.

On February 7, 2005, Slashdot reported that this exploit was disclosed at the hacker conference Shmoocon. Web browsers supporting IDNA appeared to direct the URL http://www.pа, in which the first a character is replaced by a Cyrillic а, to the site of the well known payment site Paypal, but actually led to a spoofed web site with different content.

Starting with version 7, Internet Explorer was capable of using IDNs, but it imposes restrictions on displaying non-ASCII domain names based on a user-defined list of allowed languages and provides an anti-phishing filter that checks suspicious Web sites against a remote database of known phishing sites.

On February 17, 2005, Mozilla developers announced that the next software version still has IDN support enabled, but displaying the Punycode URLs instead, thus thwarting some attacks exploiting similarities between ASCII and non-ASCII characters, while still permitting access to web sites in an IDN domain.

Since then, both Mozilla and Opera have announced that they will be using per-domain whitelists to selectively switch on IDN display for domain run by registries which are taking appropriate homograph spoofing attack precautions. As of September 9, 2005, the most recent version of Mozilla Firefox as well as the most recent Internet Explorer display the spoofed Paypal URL as "http://www.", clearly different from the original.

Safari's approach is to render problematic character sets as Punycode. This can be changed by altering the settings in Mac OS X's system files.

See also


  1. RFC 3490, IDN in Applications, Faltstrom, Hoffman, Costello, Internet Engineering Task Force (2003)
  2. Mohan, Ram, German IDN, [ German Language Table], March 2003
  3. Dam, Mohan, Karp, Kane & Hotta, IDN Guidelines 1.0, ICANN, June 2003
  4. Karp, Mohan, Dam, Kane, Hotta, El Bashir, IDN Guidelines 2.0, ICANN, November 2005
  5. What's New in Internet Explorer 7
  6. International Domain Name Support in Internet Explorer 7
  7. Handling Internationalized Domain Names (IDNs)
  8. RFC 3492, Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA), A. Costello, The Internet Society (March 2003)
  9. Mohan, Ram, GNSO IDN Working Group, Outcomes Report (PDF), ICANN
  10. On Its Way: One of the Biggest Changes to the Internet
  11. My Name, My Language, My Internet: IDN Test Goes Live
  12. Successful Evaluations of .test IDN TLDs
  13. IDNAbis overview (2008)
  14. ICANN - Paris/IDN CCTLD discussion - Wiki
  15. ICANN Seeks Interest in IDN ccTLD Fast-Track Process
  16. Proposed Final Implementation Plan: IDN ccTLD Fast Track Process, 30 September 2009
  17. Regulator approves multi-lingual web addresses, Silicon Republic, 30.10.2009
  18. NeuStar IDN details
  19. EC adopts IDN amendments to .eu regulation. News archive of The European Registry of Internet Domain Names. June 26, 2009.
  20. '.eu' internet domain to be available also in Cyrillic and Greek alphabets. Press Release. June 26, 2009.
  21. Evgeniy Gabrilovich and Alex Gontmakher, The Homograph Attack, Communications of the ACM, 45(2):128, February 2002
  22. IDN hacking disclosure by
  23. Mozilla IDN policy

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