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DNS Changes To Support IP Version 6
(Page 1 of 2)
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.
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.
IPv6 DNS Extensions
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 TypeAAAA
(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 As (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
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.
Key Concept: Even though DNS resides far above the Internet Protocol in the TCP/IP protocol suite architecture, it works intimately with IP addresses. For this reason, changes are required to allow it to support the new IPv6. These changes include the definition of a new IPv6 address resource record (AAAA), a new reverse resolution domain hierarchy, and certain changes to how messaging is performed.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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