What is ACE / IDN String?
DOMAINs with accents and umlauts are called IDN (Internationalized Domain Name). With the introduction of these IDNs, DOMAINs and mailboxes can also contain special characters outside the (pure) ASCII code. This makes it possible to use special characters such as ä, ö, ü etc.
When IDNs (internationalized domain names; domain names with umlauts) were introduced, an algorithm was defined for converting a Unicode string into a permissible ASCII DOMAIN. This ACE string (ACE = ASCII Compatible Encoding) is then entered in the DNS. In the case of an IDN, the ACE string in the DNS is not identical to the DOMAIN.
bücher.ch is the DOMAIN,
xn--bcher-kva.ch is the ACE string which is entered in the DNS.
As this procedure is unfortunately not yet supported by all browsers and mail servers, the use of umlauts in the DOMAINs and as a mail address is not yet recommended.
However, if you are considering using umlauts in your DOMAIN and mail address, please use a converter such as