The original DNS only supported ASCII letters, digits, and hyphens. Internationalized Domain Names (IDNs) extended this to allow Unicode characters — making it possible to register domains like café.com, москва.рф, or 北京.中国.
Under the hood, IDNs use a transformation called Punycode to encode Unicode as ASCII. café.com is actually stored as xn--caf-dma.com in DNS. Browsers and email clients handle the conversion transparently — you type or see the Unicode form; the network sees the Punycode form.
Practical considerations:
- Browser display rules: most browsers display IDN in Unicode form when the domain uses characters from a single script that matches the user's language. They fall back to Punycode for mixed-script domains to protect against homograph attacks (where Cyrillic
аlooks identical to Latina). - Email at IDN domains: technically supported but flaky — many older mail servers can't handle Unicode in MAIL FROM addresses. Practical advice: keep email at the Punycode form internally even if you market the Unicode version.
- SEO: search engines fully support IDNs and index both Unicode and Punycode forms.
For US/UK markets, IDNs are mostly cosmetic. For brands targeting non-Latin-script markets (Russia, China, Arabic-speaking countries, India), IDNs can be a meaningful branding tool.
