17.8. Internationalized Domain Names

One shortcoming in the original design of DNS that has become painfully obvious over the years is the character set supported in domain names. While DNS purists may tell you that labels in domain names can contain any binary value, US-ASCII characters are really the only values that are useful and supported by all DNS implementations. As the Internet has expanded internationally, this has meant that companies in countries in which European languages aren't widely spoken have been forced to use ASCII characters for their domain names. Even Europeans have had to transcribe their non-ASCII characters into ASCII; most Germans, for example, reflexively write ä and ö as ae and oe, respectively.

RFC 3490 introduced a method for encoding international characters in the labels of domain names. Because simply inserting non-ASCII characters into domain names doesn't work, most DNS software interprets multibyte characters as a sequence of ASCII characters, for example; these international characters are encoded into ASCII. The resulting ASCII-compatible encoding, or ACE, is basically unintelligible to ordinary humans in the same way that base-64 encodings are. To help distinguish an ACE-encoded label in a domain name from a normal but particularly cryptic ASCII label, ACE encodings include a specific prefix, "xn--," which is now forbidden to appear in a normal, ASCII label. Domain names with one or more labels encoded in ACE are referred to as internationalized domain names, or IDNs.