v6ptrmap - IPv6 DNS PTR Mapper

This work is based on the article "Finding v6 hosts by efficiently mapping
ip6.arpa" by Peter van Dijk which can be found at [1]

This program searches the ip6.arpa domains to find ipv6 hosts using the
assumption that DNS servers will return NOERROR when there is a domain available
in a further subdomain of the query, and NXDOMAIN when there is no further
domain available. This makes mapping these ptr records very fast compared to the
time it would take to do a linear search.

[1] http://7bits.nl/blog/2012/03/26/finding-v6-hosts-by-efficiently-mapping-ip6-arpa