IDN (Internationalized Domain Name, RFC 5890) lets domain owners use non-ASCII characters — Cyrillic, Chinese, Arabic, emoji, etc. — in labels. Over DNS these labels are transmitted as Punycode (xn--...), but browsers and applications render the original Unicode.
Enabled TLDs
Most TLDs allow IDN today. The IDN Table for each TLD defines which scripts are permitted — for example .com allows a huge range, while some ccTLDs restrict to the local script.
Use Cases
Legitimate: local-language branding in markets where Latin script is inconvenient (German umlauts, Japanese kanji, Chinese hanzi).
Malicious: homoglyph-based phishing. See the Punycode entry for details.
Browser Defence
Chrome, Firefox, and Safari display Punycode (not Unicode) when a label mixes scripts or uses characters known for homoglyph attacks — but the rules are inconsistent across browsers and versions.
Registration
You register IDNs with a normal registrar, submitting the Unicode form. The registrar handles the Punycode encoding for the registry.
Related
Check the Punycode glossary entry, the homoglyph glossary entry, and read what typosquatting is.