Editing IPv6 (section)

==IPv6 in the Domain Name System==
In the [[Domain Name System]] (DNS), [[hostname]]s are mapped to IPv6 addresses by [[AAAA record|AAAA]] ("quad-A") resource records. For [[Reverse DNS lookup|reverse resolution]], the IETF reserved the domain [[.arpa|ip6.arpa]], where the name space is hierarchically divided by the 1-digit [[hexadecimal]] representation of [[nibble]] units (4 bits) of the IPv6 address.{{Ref RFC|3596}}

When a dual-stack host queries a DNS server to resolve a [[fully qualified domain name]] (FQDN), the DNS client of the host sends two DNS requests, one querying AAAA records and the other querying A records, in that order, by default. If both types of addresses are returned by the DNS, and there is a route for it, the IPv6 address is preferred over the IPv4 address. However, the host operating system may be configured with an alternate preference for address selection.{{Ref RFC|6724}}<ref>{{Cite book|title=IPv6 Essentials: Integrating IPv6 into Your IPv4 Network|author=Silvia Hagen|publisher=O'Reilly Media, Inc.|year=2014|isbn=9781449335267|pages=176}}</ref>

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An alternative record type was used in early DNS implementations for IPv6, designed to facilitate network renumbering. The ''A6'' resource record was used for the forward lookup, completed with a number of other innovations such as ''bit-string labels'' and ''[[CNAME record#DNAME record|DNAME]]'' records.{{Ref RFC|2874}} After a discussion of the pros and cons of both schemes,{{Ref RFC|3364}}) the use of A6 resource records has been deprecated to experimental status.{{Ref RFC|3363}}

{{See also|Happy eyeballs}}