# phpman > perldoc > Net::IDN::Standards

## NAME
    [Net::IDN::Standards](https://www.chedong.com/phpMan.php/perldoc/Net%3A%3AIDN%3A%3AStandards/markdown) -- Internationalized Domain Names for Applications (IDNA)

## INTRODUCTION
    Historically, domain names and host names were restricted to a limited repertoire of ASCII
    characters, i.e. letters, digits and the hyphen (i.e. "/[A-Z0-9-]/i"). Words and names from
    languages that require additional characters (such as diacritics or special characters) or other
    scripts could not be used.

    Internationalized Domain Names (IDNs) extend the character repertoire for domain names from
    ASCII to Unicode while maintaining backwards compatibility with software that only expects and
    handles ASCII characters.

    In order to do so, Unicode domain names are converted to ASCII using an ASCII-compatible
    encoding (ACE) called Punycode. On the wire, converted domain names start with "xn--", followed
    by the ASCII encoding of the Unicode string. The Unicode version is typically only shown in
    applications presenting the domain to the user (hence Internationalized Domain Names for
    Applications, IDNA). Internationalized Resource Identifiers (IRIs), the Unicode version of URLs,
    may also include domain names in their Unicode form.

    The IDNA specifications, however, do not only cover the actual Punycode conversion but also
    include extensive rules for preparation (mapping and/or validation) of input strings. They
    typically define two functions, "ToASCII" and "ToUnicode", which prepare and convert a domain
    name to the ACE version or the Unicode version.

## DIFFERENT STANDARDS
      "The nice thing about standards is that you have so many to
      choose from."
                                           -- Andrew S. Tanenbaum

    While the actual Punycode conversion is stable, there are different specifications regarding
    mapping and/or validation (preparation):

  IDNA2003
    IDNA2003, which is defined in RFC 3490 (<<http://tools.ietf.org/html/rfc3490>>) and related
    documents, was the original specification for the internationalization of domain names.

    However, some issues were subsequently identified with IDNA2003: The specification was tied to
    Unicode 3.2 and therefore did not allow characters added in newer versions of Unicode (without
    updating the specifications).

    Furthermore, a few characters were mapped to other characters or deleted although they would
    carry meaning in some languages (i.e. 'ß' and 'ς' were mapped to 'ss' and 'σ'; ZWJ and ZWNJ were
    always mapped to nothing, although some scripts like Arabic require them for correct display).

  IDNA2008
    IDNA2008, which is defined in RFC 5890 (<<http://tools.ietf.org/html/rfc5890>>) and related
    documents, resolves the issues found in IDNA2003.

    This was done by allowing some characters that would either be mapped to other characters,
    mapped to zero and/or cause the preparation to fail. The new domain names would not be
    accessible by IDNA2003 implementations, of course.

    However, IDNA2008 also disallowed a large number of characters that had been allowed in IDNA2003
    (mostly symbols). An implementation of IDNA2008 would therefore no longer be able to access
    domain names such as "√.com", which had been registered under IDNA2003.

  UTS #46
    Unicode Technical Standard #46 (UTS #46, <<http://unicode.org/reports/tr46/>>) solves this problem
    by allowing domain names that are valid in either IDNA2003 or IDNA2008.

    This makes UTS #46 the perfect fit for domain lookup (be liberal in what you accept) but
    unsuitable for validating domain names prior to registration (be conservative in what you send).

## AUTHOR
    Claus Färber <<CFAERBER@cpan.org>>