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Internationalized Domain Names (IDN)
Frequently Asked Questions (FAQ)
What is IDN?

The abbreviation IDN stands for Internationalized Domain Name, also called a multilingual domain name. Both names are used interchangeably. An IDN is a domain name that contains characters from the Unicode character repertoire that other than letter/digit/hyphen (LDH) characters, which are Latin letters (a-z case ignored so includes A-Z), digits (0-9) and the hyphen (-). In other words, Internationalized Domain Names may contain letters with diacritics, as required by many European languages, or characters drawn from non-Latin scripts such as Arabic or Chinese.

What is IDNA?

IDNA (Internationalized Domain Names in Applications) is a mechanism devised to handle Internet domain names containing characters other than letter/digit/hyphen (LDH) characters, which are Latin letters (a-z case ignored so includes A-Z), digits (0-9) and the hyphen (-). IDNA was worked out by the IETF’s IDN Working Group. It was announced by IETF as a proposed standard in March 2003. The full concept of IDN is covered by: RFC 3490, RFC 3491, RFC 3492, RFC 3454.

Deployment of IDNA entails no changes to current Internet infrastructure and preserves the robustness of the DNS. In general, the idea behind the IDNA functioning is based on conversion of non-LDH characters of an IDN into suitable LDH ones by a user application (e.g. web browsers). Such a solution is designed for maximum backward compatibility with the existing DNS system, which supports only domains using only LDH characters. The IDNA protocol implementation does not introduce any change to the DNS infrastructure. It means that there is no need to alter any of the existing Internet’s protocols, DNS servers and resolvers on user’s computers in order to get the IDNs working. In other words, lower-layer protocols do not need to be aware of IDNs. The IDNA is a protocol of the top level layer of the OSI model, therefore the IDN introduction requires only upgrades of software which interacts with domain names, such as web browsers, e-mail and FTP clients, HTML editors, etc. In some cases, it is enough to upgrade the underlying software infrastructure, for example runtime libraries like libc, virtual machines, etc.

What is Punycode?

Punycode (RFC 3492) is an algorithm that uniquely and reversibly transforms Unicode strings into the limited character set supported by the Domain Name System. A converted IDN contains only IRA (International Reference Alphabet) characters and starts with the “xn--” prefix.

What is Unicode?

Unicode is a modern method of coding embracing characters used all over the world (e.g. German diacritics, hieroglyphs, Cyrillic), musical, technical and phonetic symbols and many more). Unicode assign the unique number - code point (independent from an OS, application or language) - to each character. Essential feature of Unicode is that the first 128 characters are equivalent to International Reference Alphabet (IRA) character set for information interchange international reference version. More information about Unicode is available at The Unicode specification is maintained by the Unicode Consortium and by ISO/IEC SC2. More on the relationship between both organizations can be found here.

What is UTF-8?

UTF-8 (8-bit Unicode Transformation Format) is a variable-length character encoding capable of representing any universal character in the Unicode standard. UTF-8 is backwards compatible with the International Reference Alphabet (IRA) character set for information interchange and uses from one to four octets per character, depending on the Unicode symbol. Many ITU-T Recommendations, ISO/IEC International Standards, IETF RFCs and ICT standards from other SDOs use the UTF-8 transformation for carrying multilingual information.

What is the International Reference Alphabet (IRA)?

The International Reference Alphabet No. 5 (formerly International Alphabet No.5, or "IA5") is an international standard specified in ITU-T Recommendation T.50 for information interchange among data processing systems and data communications systems. Each IRA character is a 7-bit coded unique character. The letters available in this character set are restricted to the Latin uppercase A-Z and lowercase a-z letters. In addition to printing characters, IRA also defines codes for control characters such as linefeed and non-printing character such as space and delete, which are the same as ANSI X3.4-1986: ASCII (American Standard Code for Information Interchange). The ASCII and ISO 646 are equivalent to the International Reference Alphabet No. 5. The advantage of IRA encoding is its (almost) universal acceptance and implementation.

Which browsers support IDN?

  • Firefox 0.6 and higher (
  • Internet Explorer 5.0 and higher + i-NavTM plug-in (
  • Internet Explorer 7 Beta 2 (
  • Mozilla 1.4 and higher (
  • Netscape Navigator 7.1 and higher (
  • Opera 7.11 and higher (
Mac OS X
  • Camino Version: 0.7 and higher (
  • Firefox 0.6 and higher (
  • Mozilla 1.4 and higher (
  • Netscape Navigator 7.1 and higher (
  • Opera 7.11 and higher (
  • Safari 1.2 and higher (
  • Epiphany 1.2.2 and higher (Gnome)(
  • 2. Firefox 0.6 and higher (
  • 3. Galeon 1.3.14 and higher (Gnome) (
  • 4. Konqueror 3.2 and higher (KDE) (
  • 5. Mozilla 1.4 and higher (
  • 6. Netscape Navigator 7.1 and higher (
  • 7. Opera 7.11 and higher (
Which e-mail clients support IDN?

  • Microsoft Outlook 2000, 2002 (XP), 2003; Outlook Express 5.0 and higher + Verisign i-Nav PlugIn (
  • Mutt 1.4.1 and higher (


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Updated : 2009-01-21