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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)

DNS Master File Format 1 2 Network File and Resource Sharing Protocols and the TCP/IP Network File System (NFS)

Version 4 of the Internet Protocol (IPv4) is the basis of today's Internet, and the foundation upon which the TCP/IP protocol suite is built. While IPv4 has served us well for over two decades, it has certain important drawbacks that would limit internetworks of the future if it were to continue to be used. For this reason, the next generation of IP, the Internet Protocol version 6 (IPv6), has been in development for many years. IPv6 will eventually replace IPv4 and take TCP/IP into the future.

The change from IPv4 to IPv6 will have effects that “ripple” to other TCP/IP protocols, including the Domain Name System. DNS is a higher-level protocol, so you might think that based on the principle of layering, a change to IP should not affect it. However, this is another example of how “strict layering” doesn't always apply. DNS works directly with IP addresses, and one of the most significant modifications that IPv6 makes to IP is in the area of addressing, so this means that using DNS on IPv6 requires some changes to how the protocol works.

Since DNS is so “architecturally distant” from IP down there at layer three, the changes required are not extensive. RFC 1886, entitled IPv6 DNS Extensions and published in December 1995, was the IETF's first formalized attempt to describe the changes needed in DNS to support IPv6. It defines three specific modifications to DNS for IPv6:

• New Resource Record Type—AAAA (IPv6 Address): The regular DNS Address resource record is defined for a 32-bit IPv4 address, so a new one was created to allow a domain name to be associated with a 128-bit IPv6 address. The four “A”s (“AAAA”) are a mnemonic to indicate that the IPv6 address is four times the size of the IPv4 address. The AAAA record is structured in very much the same way as the A record in both binary and master file formats; it is just much larger. The DNS resource record Type value for AAAA is 28.
  
  
• New Reverse Resolution Hierarchy: A new hierarchical structure similar to IN-ADDR.ARPA is defined for IPv6 reverse lookups, but the IETF put it in a different top-level domain. The new domain is IP6.INT, and is used in a way similar to how IN-ADDR.ARPA works. However, since IPv6 addresses are expressed in hexadecimal instead of dotted-decimal, IP6.INT has sixteen subdomains “0” through “F”, and each of those has sixteen subdomains “0” through “F”, and so on, sixteen layers deep. Yes, this leads to a potentially frightfully large reverse resolution database!
  
  
• Changes To Query Types And Resolution Procedure: All query types that work with A records or result in A records being included in the Additional section of a reply must be changed to also handle AAAA records. Also, queries that would normally result in A records being returned in the Additional section must return the corresponding AAAA records only in the Answer section, not the Additional section.

DNS Master File Format 1 2 Network File and Resource Sharing Protocols and the TCP/IP Network File System (NFS)