Few people but nameserver admins know the
.arpatoplevel domain. It has an hierarchical scheme with zones just as all other TLDs.
It's main use is to reverse map addresses. For an IP address likethis is done by requesting the
PTRrecord for the hostname
The DNS server for
in-addr.arpadelegates the request to the server responsible for
111.in-addr.arpaand 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.arpaand the encoding for e.g.is
But there is no technical barrier against requesting other record types from under the
.arpatree. The DNS servers happily return
DNAMEor other records when asked nicely.
And nothing prevents an DNS admin from placing non-PTR records in the
.arpasubzone. And nothing prevents them from prefixing arbitrary strings in front of the IPv6 subnet. And of course those
.arpanames can be used just like hostnames...
Perhaps URL-based filtering can be subverted this way.