Few people but nameserver admins know the
It has an hierarchical scheme with zones just as all other TLDs.
It's main use is to reverse map addresses. For an IP
this is done by
PTR record for the hostname
The DNS server for
in-addr.arpa delegates the request to
the server responsible for
and so recursively until a server is found who is responsible
for the whole network containing the address. The reply
typically is a hostname.
For IPv6 the domain is
ip6.arpa and the
encoding for e.g.
But there is no technical barrier against requesting other record
types from under the
.arpa tree. The DNS servers happily
DNAME or other records when asked nicely.
And nothing prevents an DNS admin from placing non-PTR records
.arpa subzone. And nothing prevents
them from prefixing arbitrary strings in front of the IPv6 subnet.
And of course those
.arpa names can be used
just like hostnames...
For example, a valid URL for this blog could be this or
that or even
Perhaps URL-based filtering can be subverted this way.