As of December 2017, there were 2.2 million top level IDNs (unchanged since December 2016) and 5.3 million second level IDNs (a reduction of 18% or 1.2 million since December 2016).
Top Level IDNs
Top Level (or full) IDNs have the same script at both the TLD level and the domain label.
Second Level IDNs
Second level IDNs have a non-ASCII label under an ASCII TLD.
Second level IDNs have been available since 2000, both in the ccTLD and gTLD spaces. Latin script IDNs (eg with accents and diacritics) have a natural affinity with ASCII TLDs such as .com or .eu.
It was not possible to register full ccTLD IDNs until 2009, when IDN strings were allowed into the root IANA database through the ccTLD IDN Fast Track process.
At the gTLD level, gTLD operators had to wait for ICANN to open the call for a new gTLD round to be empowered to submit applications for IDN TLDs, which occurred in 2012. Only 8% of all IDNs are top level gTLD registrations.
In December 2017, 71% of registered IDNs were at the second level, and 29% were at the top level. During 2017, the reduction in second level IDNs under .vn contributed to a drop in the proportion of second level IDNs from 75% in December 2016.