wikipedia.org:
https://en.wikipedia.org/wiki/IPv6

Internet Protocol version 6 (IPv6):
Testing both for IPv4 and IPv6, IPv6 preference is reproducable: http://www.ipv6proxy.net

This tool is like a IPv6 process. This open proxy is a forwarding proxy server that is accessible by any internet user, and is also useful for its fast propagation around the world after a changed DNS.

Analyze failures using ipv6proxy.net:

• The requested site (example.com) does not appear to have an IPv6 address.
• The requested resource could not be loaded. libcurl returned the error:
  Unknown SSL protocol error in connection to example.com:443
  – The reverse server name, IPv6 address and certificate combination may not match;
  – Reported issue 2018-09-07: probably in case of a web server without TLSv1.0.

Eg IPv6 setup for a VPS:
IP-address: 2a01:7c8:aab1:282::1
Subnet: /48
Gateway: 2a01:7c8:aab1::1

Common mistakes with IPv6:

• AAAA is always preferred over A. The application server can be limited to IPv4;
• Value, eg using ODBC, between brackets such as [2a01:7c8:aab1:282::1].

Linux statement to find out the IPv6 address see: find-ipv6-address.txt
eg 2a01:7c8:aab1:282::1 in this output:
2a01:7c8:aab1:282::1/48
2a01:7c8:aab1:282:5054:ff:fef9:cb5a/64
fe80::5054:ff:fef9:cb5a/64

or send a mail to …@gmail.com and find the IPv6 address in source code in your webmail application.

A firewall may block:
Open a not standard SSH port for IPv6 too.

An IPv6 address is changing all the time:
Stabilize login checking for security.