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Relevant RFCs and IAB Statements

IDNA2008:

Standards Track

RFC 5890 [TXT, 53 KB] Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework

RFC 5891 [TXT, 38 KB] Internationalized Domain Names in Applications (IDNA): Protocol

RFC 5892 [TXT, 183 KB] The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)

RFC 5893 [TXT, 38 KB] Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)

Informational

RFC 5894 [TXT, 113 KB] Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale

RFC 5895 [TXT, 16 KB] Mapping Characters for Internationalized Domain Names in Applications (IDNA) 2008

IAB statements

IAB Statement on Identifiers and Unicode (2018-03-15)

IAB Statement on Identifiers and Unicode 7.0.0 (2015-02-11)

Additional Relevant RFCs:

RFC 8228 [TXT, 50KB] Guidance on Designing Label Generation Rulesets (LGRs) Supporting Variant Labels

RFC 7940 [TXT, 167KB] Representing Label Generation Rulesets Using XML

RFC 6927 [TXT, 40KB] Variants in Second-Level Names Registered in Top-Level Domains

RFC 6912 [TXT, 27KB] Principles for Unicode Code Point Inclusion in Labels in the DNS

RFC 5992 [TXT, 54KB] Internationalized Domain Names Registration and Administration Guidelines for European Languages Using Cyrillic

RFC 5646 [TXT, 204KB] Tags for Identifying Languages

RFC 5564 [TXT, 23KB] Linguistic Guidelines for the Use of the Arabic Language in Internet Domains

RFC 4690 [TXT, 100 KB] Review and Recommendations for Internationalized Domain Names (IDNs)

RFC 4290 Suggested Practices for Registration of Internationalized Domain Names (IDN)

RFC 4185 [TXT, 52 KB] National and Local Characters for DNS Top Level Domain (TLD) Names

RFC 3743 [TXT, 76 KB] Joint Engineering Team (JET) Guidelines for IDN Registration and Administration for Chinese, Japanese, and Korean

RFC 1123 [TXT, 235KB] Requirements for Internet Hosts -- Application and Support

IDNA2003:

RFC 3454 [TXT, 136 KB] Preparation of Internationalized Strings ("stringprep")

RFC 3490 [TXT, 52 KB] Internationalizing Domain Names in Applications

RFC 3491 [TXT, 12 KB] Nameprep: A Stringprep Profile for Internationalized Domain Names

RFC 3492 [TXT, 68 KB] Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications

Domain Name System
Internationalized Domain Name ,IDN,"IDNs are domain names that include characters used in the local representation of languages that are not written with the twenty-six letters of the basic Latin alphabet ""a-z"". An IDN can contain Latin letters with diacritical marks, as required by many European languages, or may consist of characters from non-Latin scripts such as Arabic or Chinese. Many languages also use other types of digits than the European ""0-9"". The basic Latin alphabet together with the European-Arabic digits are, for the purpose of domain names, termed ""ASCII characters"" (ASCII = American Standard Code for Information Interchange). These are also included in the broader range of ""Unicode characters"" that provides the basis for IDNs. The ""hostname rule"" requires that all domain names of the type under consideration here are stored in the DNS using only the ASCII characters listed above, with the one further addition of the hyphen ""-"". The Unicode form of an IDN therefore requires special encoding before it is entered into the DNS. The following terminology is used when distinguishing between these forms: A domain name consists of a series of ""labels"" (separated by ""dots""). The ASCII form of an IDN label is termed an ""A-label"". All operations defined in the DNS protocol use A-labels exclusively. The Unicode form, which a user expects to be displayed, is termed a ""U-label"". The difference may be illustrated with the Hindi word for ""test"" — परीका — appearing here as a U-label would (in the Devanagari script). A special form of ""ASCII compatible encoding"" (abbreviated ACE) is applied to this to produce the corresponding A-label: xn--11b5bs1di. A domain name that only includes ASCII letters, digits, and hyphens is termed an ""LDH label"". Although the definitions of A-labels and LDH-labels overlap, a name consisting exclusively of LDH labels, such as""icann.org"" is not an IDN."