  The IPv4 address space is small enough to allow exhaustive active
measurement, permitting important insight into Internet growth, policy, and
evolution. The IPv6 address space, on the other hand, presents the problem that
we can no longer perform exhaustive measurements in the same way, inhibiting
our ability to continue studying Internet growth. Access to private datasets
(e.g., HTTP access logs on content servers, flow data in ISP networks, or
passive DNS traces) solves some problems but may not be feasible or desirable.
This paper describes IPv6 address collection by exhaustively sweeping the
reverse DNS domain for the IPv4 address space and performing AAAA queries on
the results. Subsequent ICMP and TCP measurements are conducted to measure the
responsiveness of the resulting set. Key outcomes include: the PTR sweep
discovers 965,304 unique, globally routable IPv6 addresses originating from
5,531 ASNs. 56% of the addresses are responsive, across 4,571 ASNs. Upon
inferring pairs of IPv4 and IPv6 addresses that are likely associated with the
same device, the data indicates a trend toward IPv4 addresses being more
responsive than their IPv6 counterparts, with a higher incidence rate of TCP
connections being refused, and wide disparity on where TCP connections or ICMP
echo requests fail silently when comparing IPv4 and IPv6. The disparity in IPv4
and IPv6 responsiveness is highly variable, and indicative of distinct host
configuration and network policies across the two networks, presenting
potential policy or security gaps as the IPv6 network matures.
