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An ASCII-based domain name is written using ASCII characters, but that does not automatically make it an internationalized domain name (IDN). In IDNA terminology, an IDN label has a Unicode form called a U-label and an ASCII-compatible DNS form called an A-label. Ordinary ASCII labels, such as those in icann.org, do not need the xn-- prefix.
What is an ASCII-based domain name?
A domain name is made up of labels separated by dots. For example, icann.org has the labels icann and org. The ASCII-versus-Unicode distinction applies to each label, rather than to the dotted name as a single encoded unit. ICANN explains the distinction between ASCII and Unicode forms of IDN labels.
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An ordinary ASCII label commonly uses letters, digits and hyphens—the LDH character set. A domain made only of these ASCII labels is an ASCII domain, but it is not an IDN merely because it is written in ASCII. ICANN uses icann.org as an example of a name that is not an IDN. ICANN’s IDN overview
How an ASCII label differs from an IDN A-label
| Form | Characters and role | Example or identifier |
|---|---|---|
| Ordinary LDH label | ASCII letters, digits and hyphens; an ordinary ASCII label, not necessarily an IDN. | icann in icann.org; it does not require xn--. ICANN |
| U-label | The Unicode form of an IDNA-valid internationalized label. RFC 5890 specifies that a U-label is in NFC and contains at least one non-ASCII character. | Unicode representation of an internationalized label. RFC 5890 |
| A-label | The ASCII-Compatible Encoding (ACE) form corresponding to an IDNA-valid U-label, used for DNS operations. | A-labels begin with the xn-- prefix. RFC 5890 |
What does the xn-- prefix mean?
The prefix identifies an A-label: an ASCII-compatible representation of an internationalized label. It is not a marker required on every ASCII domain label. An ordinary label such as icann can be ASCII without beginning with xn--. ICANN describes ACE, commonly associated with Punycode, as a way to represent an internationalized label in ASCII for DNS use. ICANN: ASCII-compatible encoding (ACE)
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An A-label represents a complete label, not an arbitrary piece of text within a label. The corresponding Unicode form is its U-label. The two are related representations of an IDN label: the U-label uses Unicode, while the A-label supplies its ASCII-compatible DNS representation. RFC 5890
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Is a Punycode domain an IDN?
When “Punycode domain” refers to a domain containing an A-label, it is describing the ASCII-compatible representation of an IDN label. The label is internationalized even though its DNS-facing form is ASCII. This differs from an ordinary ASCII-only domain such as icann.org, which is not an IDN just because ASCII is used. ICANN’s IDN overview
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Keep the terminology straight
- ASCII domain: a domain name whose labels are represented using ASCII characters; this description alone does not mean the domain is an IDN.
- LDH label: an ordinary ASCII label using letters, digits and hyphens.
- U-label: the Unicode form of an IDNA-valid internationalized label, with at least one non-ASCII character and NFC normalization under RFC 5890.
- A-label: the ACE form of an IDNA-valid U-label; it is a complete label and begins with
xn--.
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