On Linux, IPv4 & IPv6 run side by side.
If a site has an IPv6 address only, that address will be used.
If a site has both IPv6 and IPv4 addresses, the IPv6 address will be used.
If a site has an IPv4 address only, that address will be used.
It is totally transparent. The only way you know if you are using IPv6 or IPv4 to contact a site is if the site tells you. See noc.enta.net
I have not encountered any sites that I could access when I had only IPv4, that I cannot access now that I have both IPv6 and IPv4. (when using BT 21CN connectivity)
This was not the case when using BT 20CN connectivity. As I recall this was due to a problem with some BT Cisco equipment not handling small packets. This effected ipv6.google.com.
Another reason that people may not be able to access certain sites is, if the authoritative dns server for a site is contactable on IPv6 only, and their ISP's dns servers can only contact other dns servers using IPv4.
Then the addresses for those sites will be unsresolvable.
I only know of one site that I cannot access because of this. (I use Enta's dns servers)
If you know of any specific sites that people are having problems with, let me know what they are and I will try them.
UKFSN since April 2004.