IDN: definition

An internationalized domain name (IDN) uses more than the letters a to z, the digits and the hyphen: accented letters such as ñ or ü, or scripts such as Arabic, Chinese or Cyrillic. People see and type the name in their own writing, while the DNS works with a plain-letter version behind the scenes. Most IDN problems come from the gap between those two forms.

What an internationalized domain name is

RFC Request for Comments A numbered document in the series recording the internet's technical standards and practices. Full definition of RFC 5890, published by the IETF Internet Engineering Task Force The open body that writes internet standards such as DNS, EPP and RDAP. Full definition of IETF in August 2010, defines an IDN as a domain name with at least one internationalized label One dot-separated part of a domain name, up to 63 characters long. Full definition of label. That label can be the registered name, such as a name with ñ under .es, or the top-level domain itself, such as .рф.

How it works behind the scenes: Punycode, A-labels and U-labels

IDNs work one label at a time, never on the whole name. Each internationalized label has two forms:

The U-label The human-readable Unicode form of one part of an internationalized domain name. Full definition of U-label is the form people read and type, in Unicode characters.
The A-label is its ASCII equivalent: the prefix xn– followed by the output of the Punycode algorithm, which can be at most 59 characters, since a label is at most 63.

Software converts each U-label into its A-label before querying the DNS: in ICANN Internet Corporation for Assigned Names and Numbers The nonprofit that coordinates the global DNS and gTLD policy. Full definition of ICANN’s words, DNS operations “use A-labels exclusively”. RFC 5890 keeps the word Punycode for the conversion method (RFC 3492, March 2003), not for the resulting string.

The current rule set, IDNA2008 Internationalized Domain Names for Applications, 2008 revision The current standard defining which characters internationalized domain names may use. Full definition of IDNA2008 (RFCs 5890 to 5894), replaced IDNA2003 but kept Punycode and the xn– prefix. A few characters changed: the German ß and the Greek final ς are valid in IDNA2008, while IDNA2003 mapped them to other characters, so a name containing them now has a different A-label from the one older software looks up.

Which characters each registry allows

IDNA2008 sets the outer limit. Within it, RFC 5890 makes restrictions mandatory for IDN registry The central database and system of a top-level domain, or loosely the organization that runs it. Full definition of registry, and RFC 5891 expects registries at every level of the DNS to set their own policies. Many publish IDN table A registry's list of the characters it allows in internationalized domain names. Full definition of IDN table, lists of the characters they accept for a language or script, which IANA Internet Assigned Numbers Authority The functions that coordinate the root zone, IP addresses and protocol numbers. Full definition of IANA collects in its Repository of IDN Practices.

LGR Label Generation Rules Machine-readable rules defining valid labels and their variants for a zone. Full definition of LGR (LGR) are their formal, machine-readable version (RFC 7940). For top-level domains, ICANN uses the Root Zone Label Generation Rules (RZ-LGR). As of October 2026, the current version, RZ-LGR-6, covers 27 scripts, Latin among them.

For .es, the national plan for domain names (Orden ITC/1542/2005) and Red.es The Spanish public body that manages the .es domain through its unit Dominios.es. Full definition of Red.es set these rules, as of October 2026:

the letters a to z, the digits 0 to 9 and the hyphen, never as first or last character;
13 more characters: á, à, é, è, í, ï, ó, ò, ú, ü, ñ, ç and l·l;
3 to 63 characters at the second level, 2 to 63 at the third;
no name may start with xn–, and capitals count as lowercase.

For the Catalan l·l, IDNA2008 allows the middle dot only between two l’s, not the single letter ŀ.

Variants and look-alike characters

Some script communities, such as Arabic and Han users, treat two different strings as the same name. These are IDN variant A label treated as equivalent to another because its characters are interchangeable. Full definition of IDN variant.

Look-alikes are a different problem: characters from different scripts that look the same, such as a Greek omicron among Latin letters. A homograph attack Using look-alike characters to make a fake domain look real. Full definition of homograph attack uses them to make a fake name look real. Registry restrictions and LGRs limit such mixes, and ICANN’s string similarity evaluation A check of whether a proposed TLD string could be confused with another one. Full definition of string similarity evaluation rejects applied-for TLD top-level domain The last part of a domain name, after the final dot. Full definition of TLD that are visually similar to existing ones.

IDN top-level domains in non-Latin scripts

IDN country code TLDs come through the IDN ccTLD Internationalized Domain Name country code top-level domain A country code TLD written in a non-Latin script. Full definition of IDN ccTLD Fast Track, approved by the ICANN Board ICANN's board of directors, which adopts policies and approves key decisions. Full definition of ICANN Board in Seoul in October 2009. A country or territory on the ISO 3166-1 The standard list of country codes that decides which ccTLDs can exist. Full definition of ISO 3166-1 list may ask for its name in a script other than Latin, so .es is outside it.

Generic IDN TLD Internationalized Domain Name Top-Level Domain A top-level domain written in a non-Latin script or with accented Latin letters. Full definition of IDN TLD come through ICANN’s new gTLD A generic top-level domain added to the internet through ICANN's New gTLD Program. Full definition of new gTLD rounds. In the 2026 round, an applied-for IDN must comply with IDNA2008 and RZ-LGR-6. A variant goes to the same registry operator The organization that holds the contract or mandate for a TLD and sets its rules. Full definition of registry operator under one Registry Agreement The contract between ICANN and a gTLD registry operator. Full definition of Registry Agreement, and each variant needs a justification. As of October 2026, operators of existing gTLD generic top-level domain A top-level domain not tied to a country, run under ICANN contracts. Full definition of gTLD pay no base application fee for up to four variant strings.

As of October 2026, about 150 TLDs in IANA’s root zone The top of the DNS, listing every TLD and its name servers. Full definition of root zone database start with xn– (our count), and ICANN cites more than 60 IDN ccTLDs. Examples include .рф (xn–p1ai), the country code TLD for Russia, and the generic .vermögensberater (xn–vermgensberater-ctb).

Where IDNs still break: email, apps and forms

ICANN calls the goal that every valid domain name and email address works in all software Universal Acceptance The goal that every valid domain name and email address works in all software. Full definition of Universal Acceptance. Forms that reject accented letters are a typical failure.

Email needs separate standards, Email Address Internationalization (EAI), introduced in RFC 6530 in February 2012. When multilingual .es names opened in 2007, Red.es warned that IDN email was uncommon and that some software would show the xn– form. That was the 2007 position.

Example: registering a Spanish name with ñ

Take the illustrative name peña.example: software turns the label peña into xn–pea-8ma, so the DNS sees xn–pea-8ma.example. Because .example is reserved, this shows only the conversion. In Red.es’s own published example, eñe becomes xn–ee-zja.

For a .es name with ñ:

Check that it uses only the allowed characters and lengths above.
Register it through a .es registrar A company that registers domain names for customers with the registry. Full definition of registrar, which sets its own price. Red.es registers directly only in very specific cases; as of October 2026, its direct price is 33.38 euros a year, value added tax included, with no separate price for multilingual names.

The registry then records the name, and the DNS works with its A-label. The sources consulted do not say whether it is linked to the same name without ñ.

IDN