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 setup for a VPS:
IPv6 address such as 2a01:7c8:d008:32:5054:ff:fee8:665a
The gateway (device on a network) is a single IP within an IP block such as 2a01: 7c8: d008 :: 1.

Common mistakes with IPv6:

• AAAA (IPv6) is always preferred over A (IPv4).
  The application server can be limited to IPv4 (can come to a dead end);
• Value, eg using ODBC, between brackets such as [2a01:7c8:aab1:282::1].
  (I stabilize ODBC with an IPv4 address instead of a domain name)

The meanwhile criticized ‘ifconfig’ statement to find out the IPv6 address:
$ ifconfig |grep inet6
eg:
inet6 2a01:7c8:d008:32:5054:ff:fee8:665a prefixlen 64 scopeid 0x0
inet6 fe80::5054:ff:fee8:665a prefixlen 64 scopeid 0x20
inet6 ::1 prefixlen 128 scopeid 0x10

A firewall may block:
In case of non-standard SSH port, open for IPv6 as well.

The IPv6 address of the internet connection can change e.g. every 15 minutes:
Stabilize programming code in case of a security login check .