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Punycode
Converts internationalized domain names to punycode (xn--…) and back.
scheme and path are stripped — only the hostname is converted
Internationalised domain names
DNS carries ASCII only, so a domain with non-Latin characters is transported in its Punycode form, an xn-- prefixed label. This converts in both directions, per label, exactly as a resolver does.
It is the form you need when a certificate, a DNS record or a log line shows xn--e1afmkfd.xn--p1ai and you need to know which domain that is.
Why does each part of the domain convert separately?
Punycode operates per label, between the dots. That is why a mixed domain can have one xn-- label and one plain ASCII label — and why converting the whole string as one unit gives the wrong answer.
What is a homograph attack?
Registering a name that looks identical to a known brand using characters from another script — Cyrillic а instead of Latin a, for example. The two are different domains with different Punycode. Converting a suspicious link is the quickest way to see which one you are actually looking at.
Should I put the Unicode or the Punycode form in my DNS records?
Punycode. Most control panels convert for you, but certificates, SPF includes and CAA records are matched byte for byte — a Unicode form there simply does not match.
Related tools: DNS lookup, WHOIS lookup and Base64.