How to Encode Punycode Domains

Problem

You have a domain name with non-ASCII characters (e.g., Bücher.com) and need its Punycode form for DNS lookups.

Prerequisites

• Understanding of DNS
• Familiarity with Unicode

Steps

1. 1. Split the domain into labels

   Separate the domain name at each dot. Punycode is applied per-label, not to the whole domain. The TLD and each subdomain label are processed independently.

   Input

   Bücher.com

   Output

   labels: ['Bücher', 'com']

2. 2. Encode each non-ASCII label to Punycode

   For each label containing non-ASCII characters, prefix the Punycode output with xn-- and append the encoded form. ASCII-only labels like 'com' remain unchanged. The encoding maps Unicode code points to a compact ASCII representation.

   Input

   Bücher

   Output

   xn--Bcher-kva

3. 3. Reassemble and verify

   Join the encoded labels with dots. Use the result for DNS resolution. Most browsers and DNS libraries perform this conversion automatically, but APIs and logs often show the Punycode form.

   Input

   xn--Bcher-kva.com

   Output

   resolves to the same server as Bücher.com

Expected result

An ASCII-only domain name (ACE form) that DNS resolvers can query, equivalent to the original IDN.

Common errors

Encoding the entire domain as one label

Split on dots first. Only labels with non-ASCII characters get the xn-- prefix.

Case is lost in conversion

Punycode preserves the case of the first character. Use a standards-compliant encoder to handle case correctly.

Troubleshooting

• If DNS lookup fails, verify the xn-- prefix is present and lowercase.
• Some older resolvers do not support IDN. Use the Punycode form directly in those cases.

Security limits

• Punycode enables homograph attacks (e.g., аpple.com using Cyrillic а). Always display the Unicode form to users and warn about mixed-script domains.