www.example.com
is an FQDN.a
to z
, 0
to 9
and the hyphen (-
). They can for example contain Arabic, Chinese, Cyrillic, Tamil, Hebrew or Latin alphabet-based characters with diacritics or ligatures. The encoded representation of an IDN domains starts with xn--
. IDNs are supported by Let’s Encrypt: https://letsencrypt.org/2016/10/21/introducing-idn-support.html. Wikipedia - RFC 5890 - RFC 5891com
and co.uk
are Public Suffixes even though co.uk
is not a TLD. Web browsers use the list, among other things, for preventing sites that are likely operated by different entities from sharing web cookies with one another. Let’s Encrypt also uses the list for rate-limit calculations: /docs/rate-limits. https://publicsuffix.org/.de
(Germany), .cn
(China) and generic top-level domains (gTLDs) like .com
, .org
. Wikipedia*.example.com
is valid for blog.example.com
and www.example.com
but not for bork.bork.example.com
or example.com
). A wildcard is indicated by an asterisk character (*) in place of a subdomain. Let’s Encrypt provides Wildcard certificates as of March 2018. Wikipedia