The four-character prefix that begins a Punycode domain label is xn--. It signals that the rest of the label is an ASCII-compatible encoding of a Unicode domain name.
Domain names were originally designed around a limited ASCII character set. Internationalized domain names allow scripts such as Arabic, Cyrillic, Greek, and Chinese, as well as some emoji, to appear in addresses. DNS infrastructure still handles an ASCII form, so the visible Unicode label is converted into an “A-label” beginning with xn--.
The characters after the prefix contain the Punycode payload. For example, the emoji 😉 can be represented as xn--n28h, while an emoji-containing label such as i❤ becomes xn--i-7iq. The prefix itself does not identify a particular language or emoji; it identifies the encoding format.
The prefix is often confused with part of the domain’s original name, but it is technical syntax added for DNS compatibility. Browsers may show either the Unicode form or its xn-- representation depending on security rules and browser behavior.