Today is World IPv6 Day, with the tag-line "This time it is for real". This follows on from a successful results of a test last year.
The effect will be that large-scale websites, such as Google, YouTube, Facebook and others will enable IPv6 address records for their main sites, as well as the existing IPv4 addresses they already have.
IPv4 addresses are stored in DNS as A records, which consist of 4-byte (32-bit) addresses in dotted decimal such as 63.246.7.184. IPv6 addresses are stored as AAAA records, which consist of a 16-byte (128-bit) address, distinguished from IPv4 addresses as they use hexadecimal numbers and colons as the separators. For example, Facebook's allocated subnet is 2a03:2880:2110:3f03:face:b00c::.
Most operating systems can natively support IPv6 already, and when a DNS lookup is resolved it will give back answers for both A and AAAA records, if available. Operating systems will prefer an IPv6 connection if available, and fallback to an IPv4 connection if not.
Statistics collected by Google show that the majority of users have no problems when connecting to a dual-stack host. However, for those with incorrectly configured IPv6 routers, the initial delay whilst resolving to a site may be less desirable. As a result, newer web browsers such as Chrome will perform a dual connection upon initial resolve, and prefer the faster of the two connections (which may be over IPv4). By providing this mechanism, the fastest site to respond is given priority over future connections to that host.
With few users on IPv6, the introduction of IPv6 addresses everywhere is unlikely to have a significant detrimental impact. It will, however, give a starting place for a wider adoption of IPv6 on the client side, which becomes necessary as the IPv4 addresses have already run out of the IANA allocated address blocks.
You can test your IPv6 connectivity by visiting Test IPv6 website.