An internationalized domain name (IDN) is a domain name that can include non-ASCII characters from supported writing systems, such as Cyrillic, Arabic, or accented Latin letters. Because the DNS still relies on ASCII-compatible labels, IDNs are encoded for transport, often with Punycode, and then rendered back to readable text by the browser or resolver.
IDNs matter in cyber security because they can be abused in homograph attacks, where visually similar characters are used to mimic a trusted brand or login page. A lookalike domain may appear legitimate in the address bar, making phishing easier and visual inspection less reliable. Defenders reduce this risk by validating domains programmatically, showing Punycode where appropriate, using allowlists, and training users not to trust appearance alone.