LDH

LDH, or Letter, Digit, Hyphen, refers to the Domain Name System's (DNS) hostname rule that restricts domain names to the use of letters (a-z), numbers (0-9), a hyphen (-) and a period (.) prior to internationalization of the DNS. The period is only allowed as a delimiter to separate different levels of a domain name.

Introduction of IDNs

With the Introduction of Internationalized Domain Names in Application (IDNA) as the standard for IDNs, the LDH restriction no longer applies to domain names. IDNA makes this possible by using separate labels for display to end user and DNS processes.

U-Labels

IDNs are displayed to the end user using a Unicode encoding scheme. This is known as the U-Label.

Example:

The U-Label for .бг, the country code top-level domain for Bulgaria, contains the Unicode codepoints U+0431 (б) and U+0433 (г).

A-Labels

Wt the DNS level, IDNs are processed using an ASCII-compatible encoding scheme. This is known as the A-Label.

Example:

The A-label for .бг is xn--90ae.