IDN (Internationalized Domain Names) are international domain names that may contain characters of national alphabets: Cyrillic, Arabic, Chinese, Japanese and others.

How IDN Domains Work

The browser shows the domain in Cyrillic letters. Internally the system converts it to the Punycode format that DNS servers understand:

Cyrillic "primer.rf"    →  xn--e1afmapc.xn--p1ai
Cyrillic "ukrayina.ua"  →  xn--80aaxfb7a.ua

DNS servers work only with Punycode. Punycode always starts with xn--.

Advantages of IDN Domains

Easy to remember for an audience that does not use the Latin alphabet
Branding in the native language
Zones such as .рф (Russia, punycode xn--p1ai), the Ukrainian Cyrillic zone xn--j1amh and the Chinese xn--fiqs8s are available

Drawbacks and Risks of IDN Domains

Homograph attacks — Cyrillic characters look like Latin ones (a vs a). Attackers register phishing domains
Email problems — not every mail client supports IDN
SEO difficulties — Google indexes the Punycode version, which can be confusing
Compatibility — older systems do not understand IDN

⚠️ Phishing through IDN: The letter "a" in Cyrillic and in Latin look identical. Fraudsters register domains with Cyrillic characters that resemble well-known brands. Be careful when following links.

Does Your Site Need an IDN Domain

Situation	Recommendation
Local business for a Ukrainian audience	Better .ua or .com.ua in Cyrillic
International project	Only .com — IDN creates problems
Brand protection	You can register an IDN version with a redirect

💡 In practice: Most successful Ukrainian sites run on .ua or .com.ua with Latin characters. IDN domains are a niche tool, not a replacement for standard domains.

How the Punycode ACE Prefix Is Built

Punycode is the algorithm from RFC 3492 that encodes non-ASCII characters into a string made only of Latin letters, digits and a hyphen. The resulting label gets the xn-- prefix (ACE — ASCII Compatible Encoding), followed by the encoded part. DNS servers, registries and most registrars work only with this ASCII form — the native-script spelling exists only at the browser and control-panel level.

# Converting to Punycode and back (libidn2 package, idn2 tool)
idn2 primer.rf
# xn--e1afmapc.xn--p1ai

idn2 --decode xn--e1afmapc.xn--p1ai
# primer.rf

One subtlety: the same visible text can be encoded differently depending on the IDNA table version (IDNA2003 vs IDNA2008). Domains registered before a registry moved to IDNA2008 sometimes produce a different Punycode string for characters such as the German "ß" or certain letters with diacritics — when transferring such a domain to another registrar it is worth comparing the ACE string character by character rather than trusting a visual match.

Common Errors With IDN Domains

Below are the messages people actually run into when registering and maintaining IDN domains, and what is behind them.

Message or symptom	Cause	Fix
"Invalid domain name format" in the registrar's form	A native-script name was typed directly into a field that expects ASCII, with no client-side conversion to Punycode	Convert the domain with idn2 or an online converter and enter the resulting xn--... form
dig returns NXDOMAIN for the native-script spelling	The query went to the resolver as raw UTF-8 instead of the ACE form — no zone exists under that literal name	Query only the Punycode form: `dig xn--e1afmapc.xn--p1ai`
A message is rejected with "550 5.1.1 Recipient address rejected"	The part before the "at" sign is in a non-Latin script, and the recipient's mail server does not support SMTPUTF8/EAI (RFC 6531)	Use a Latin mailbox name on the Punycode domain instead of a non-ASCII local part
The browser shows the bare xn--... string instead of the native script	The browser's anti-phishing filter detected mixed scripts in the label and deliberately disabled Unicode display	This is normal homograph protection, not a domain configuration error — there is nothing to fix

The last row is the most common support request: the owner is sure the domain is "broken", while in fact the browser's own protection kicked in, and it has no effect on resolution or mail delivery.

Checking an IDN Domain

# DNS and WHOIS must be queried only in the Punycode form
dig xn--e1afmapc.xn--p1ai A +short
whois xn--e1afmapc.xn--p1ai

# Confirm the registrar returned the expected ACE prefix
idn2 primer.rf

If whois on the Punycode form returns "No match for domain" while the native-script spelling appears "taken" at another registrar, it is most likely two different domains in different TLDs, or a mistake in manually building the ACE string, not a genuine conflict.

When an IDN Domain Does Not Fit

An international B2B project where the main audience types the address on a Latin keyboard layout
You need email addresses for external partners — some corporate mail servers still reject EAI addresses with a non-ASCII local part
The domain has to be read aloud over the phone in a call center — a Punycode string cannot be dictated by voice
There is no capacity to continuously monitor similar homograph registrations by competitors or scammers
The main traffic channel is offline advertising and signage, where the address is typed in manually letter by letter

For an ordinary project with no need for a non-Latin script, comparing zones is covered in how to choose a domain zone.