TCP/IP Tutorial and Technical Overview - IBM Redbooks

12.1.6 Mapping IP addresses to domain names: Pointer queries
The Domain Name System provides for a mapping of symbolic names to IP
addresses and vice versa. While the hierarchical structure makes it easy in
principle to search the database for an IP address using its symbolic name, the
process of mapping an IP address to a symbolic name cannot use the same
process. Therefore, there is another namespace that facilitates the reverse
mapping of IP address to symbolic name. It is found in the domain in-addr.arpa
(arpa is used because the Internet was originally the ARPAnet).
Not including IPv6, IP addresses are normally written in dotted decimal format,
and there is one layer of domain for each hierarchy. Contrary to domain names,
which have the least-significant parts of the name first, the dotted decimal format
has the most significant bytes first. Therefore, in the Domain Name System, the
dotted decimal address is shown in reverse order.
For example, consider the following IPv4 address:
129.34.139.30
The in-add.arpa address for this is:
30.139.34.129.in-addr.arpa.
This is handled slightly different for IPv6 addresses. Because of the IPv6
address’ structure, the reverse order is done in nibbles in stead of octets. Also,
the in-addr.arpa domain does not include IPv6. Instead, the domain used is
IP6.ARPA. For example, consider the following IPv6 address:
4321:0:1:2:3:4:567:89ab
Breaking this into nibbles, reversing the odder, and appending the domain yields:
b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.IP6.ARPA
Given an IP address, the Domain Name System can be used to find the
matching host name. A domain name query to do this is called a pointer query.
12.1.7 The distributed name space
The Domain Name System uses the concept of a distributed name space.
Symbolic names are grouped into zones of authority, more commonly referred to
as zones. In each of these zones, one or more hosts has the task of maintaining
a database of symbolic names and IP addresses within that zone, and provides a
server function for clients who want to translate between symbolic names and IP
addresses. These local name servers are then (through the internetwork on
which they are connected) logically interconnected into a hierarchical tree of
domains. Each zone contains a part or a subtree of the hierarchical tree, and the
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