"Introduction This document supplies the protocol definition for Internationalized Domain Names in Applications (IDNA)@ with the version specified here known as IDNA2008. Essential definitions and terminology for understanding this document and a road map of the collection of documents that make up IDNA2008 appear in a separate Definitions document [RFC5890]. Appendix A discusses the relationship between this specification and the earlier version of IDNA (referred to here as ""IDNA2003""). The rationale for these changes@ along with considerable explanatory material and advice to zone administrators who support IDNs@ is provided in another document@ known informally in this series as the ""Rationale document"" [RFC5894]. IDNA works by allowing applications to use certain ASCII [ASCII] string labels (beginning with a special prefix) to represent non-ASCII name labels. Lower-layer protocols need not be aware of this; therefore@ IDNA does not change any infrastructure. In particular@ IDNA does not depend on any changes to DNS servers@ resolvers@ or DNS protocol elements@ because the ASCII name service provided by the existing DNS can be used for IDNA. IDNA applies only to a specific subset of DNS labels. The base DNS standards [RFC1034] [RFC1035] and their various updates specify how to combine labels into fully-qualified domain names and parse labels out of those names. This document describes two separate protocols@ one for IDN registration (Section 4) and one for IDN lookup (Section 5). These two protocols share some terminology@ reference data@ and operations."