Re: need? really?
That's not how it works.
ISPs provide v6 and v4 in parallel allocating both a v4 address, for which the customer is generally obliged to supply NAT for themselves, as well as an IPv6 prefix so that the customer's computers can configure themselves with SLAAC if they haven't explicitly set up DHCPv6.
All consumer operating systems are smart enough to send both AAAA and A requests when they do DNS lookups, so when I talk to google on my laptop here I get their v6 addresses, but when I talk the register (who still have no v6 configured, SHAME) I get the v4. v6 traffic goes out over the ADSL in v6 records, v4 goes out in v4 records and never the twain shall meet. The ISP provides no 6to4 gateway, and I'm pretty sure if I tried to ping IPv4 address in the IPv6, my ISP would just drop the packets.