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An ASCII-based domain name is written using characters from the ASCII character set. In domain-name terminology, it helps to distinguish an ordinary ASCII-only name, such as icann.org, from the ASCII representation of an internationalized domain name (IDN), called an A-label.
How domain names are divided
A domain name consists of labels separated by dots. In icann.org, for example, icann and org are separate labels. The distinction between an ordinary ASCII label and an IDN’s ASCII representation applies to each label, not to the dotted name as one indivisible string. ICANN’s IDN overview describes this distinction.
Ordinary ASCII labels and A-labels
| Form | Characters and role | Example or marker |
|---|---|---|
| Ordinary LDH label | Uses ASCII letters, digits, and hyphens. It is an ordinary ASCII label, not an IDN just because it is written in ASCII. | icann.org is ICANN’s example of an ASCII-only domain. |
| A-label | The ASCII-Compatible Encoding (ACE) form of an IDNA-valid Unicode label. It represents an internationalized label in ASCII for DNS use. | Begins with xn--. |
| U-label | The Unicode form of an internationalized label. Under RFC 5890, a U-label is IDNA-valid, is in NFC, and contains at least one non-ASCII character. | The Unicode label paired with its A-label. |
These definitions come from ICANN and RFC 5890, IDNA Definitions.
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What happens with an internationalized label?
An IDN label can have a Unicode form that people recognize and an ASCII-compatible form used for DNS operations. The Unicode form is the U-label; the ASCII form is the A-label, which uses the ACE prefix xn--. ICANN describes ACE/Punycode as the method for representing an internationalized label in ASCII for DNS use: ASCII-compatible encoding (ACE).
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The prefix identifies an A-label; it is not required on every ASCII label. A standard ASCII-only name such as icann.org is not an IDN merely because it consists of ASCII characters. Nor is xn-- an arbitrary tag for any substring: an A-label is a complete label defined by IDNA.
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In brief
- ASCII-based domain names use ASCII characters, but that description alone does not mean a name is an IDN.
- Ordinary LDH labels use letters, digits, and hyphens.
- For an internationalized label, the Unicode form is a U-label and its ASCII-compatible DNS form is an A-label beginning with
xn--. - Because these terms describe labels, do not treat IDNA as a single conversion applied to an entire dotted domain name.
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