  Today's network measurements rely heavily on Internet-wide scanning,
employing tools like ZMap that are capable of quickly iterating over the entire
IPv4 address space. Unfortunately, IPv6's vast address space poses an
existential threat for Internet-wide scans and traditional network measurement
techniques. To address this reality, efforts are underway to develop
``hitlists'' of known-active IPv6 addresses to reduce the search space for
would-be scanners. As a result, there is an inexorable push for constructing as
large and complete a hitlist as possible.
  This paper asks: what are the potential benefits and harms when IPv6 hitlists
grow larger? To answer this question, we obtain the largest IPv6 active-address
list to date: 7.9 billion addresses, 898 times larger than the current
state-of-the-art hitlist. Although our list is not comprehensive, it is a
significant step forward and provides a glimpse into the type of analyses
possible with more complete hitlists.
  We compare our dataset to prior IPv6 hitlists and show both benefits and
dangers. The benefits include improved insight into client devices (prior
datasets consist primarily of routers), outage detection, IPv6 roll-out,
previously unknown aliased networks, and address assignment strategies. The
dangers, unfortunately, are severe: we expose widespread instances of addresses
that permit user tracking and device geolocation, and a dearth of firewalls in
home networks. We discuss ethics and security guidelines to ensure a safe path
towards more complete hitlists.
