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	English
	The DNS is mainly used to find the address that corresponds to a given name. However, it is sometimes useful to obtain the name that corresponds to an IP address. This done by using the `PTR` (`pointer`) `RR`. The `RData` part of a `PTR` `RR` contains the name while the `Name` part of the `RR` contains the IP address encoded in the `in-addr.arpa` domain. IPv4 addresses are encoded in the `in-addr.arpa` by reversing the four digits that compose the dotted decimal representation of the address. For example, consider IPv4 address `192.0.2.11`. The hostname associated to this address can be found by requesting the `PTR` `RR` that corresponds to `11.2.0.192.in-addr.arpa`. A similar solution is used to support IPv6 addresses :rfc:`3596`, but slightly more complex given the length of the IPv6 addresses. For example, consider IPv6 address `2001:1890:123a::1:1e`. To obtain the name that corresponds to this address, we need first to convert it in a reverse dotted decimal notation : `e.1.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.a.3.2.1.0.9.8.1.1.0.0.2`. In this notation, each character between dots corresponds to one nibble, i.e. four bits. The low-order byte (`e`) appears first and the high order (`2`) last. To obtain the name that corresponds to this address, one needs to append the `ip6.arpa` domain name and query for `e.1.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.a.3.2.1.0.9.8.1.1.0.0.2.ip6.arpa`. In practice, tools and libraries do the conversion automatically and the user does not need to worry about it.
	An important point to note regarding the Domain Name System it that it is extensible. Thanks to the `Type` and `RDLength` fields, the format of the Resource Records can easily be extended. Furthermore, a DNS implementation that receives a new Resource Record that it does not understand can ignore the record while still being able to process the other parts of the message. This allows, for example, a DNS server that only supports IPv6 to safely ignore the IPv4 addresses listed in the DNS reply for `www.ietf.org` while still being able to correctly parse the Resource Records that it understands. This allowed the Domain Name System to evolve over the years while still preserving the backward compatibility with already deployed DNS implementations.
	Footnotes
	Some DNS resolvers allow any host to send queries. Google operates a `public DNS resolver <https://developers.google.com/speed/public-dns/docs/using>`_ at addresses `2001:4860:4860::8888` and `2001:4860:4860::8844`. Other companies provide similar services.