  There is striking volume of World-Wide Web activity on IPv6 today. In early
2015, one large Content Distribution Network handles 50 billion IPv6 requests
per day from hundreds of millions of IPv6 client addresses; billions of unique
client addresses are observed per month. Address counts, however, obscure the
number of hosts with IPv6 connectivity to the global Internet. There are
numerous address assignment and subnetting options in use; privacy addresses
and dynamic subnet pools significantly inflate the number of active IPv6
addresses. As the IPv6 address space is vast, it is infeasible to
comprehensively probe every possible unicast IPv6 address. Thus, to survey the
characteristics of IPv6 addressing, we perform a year-long passive measurement
study, analyzing the IPv6 addresses gleaned from activity logs for all clients
accessing a global CDN.
  The goal of our work is to develop flexible classification and measurement
methods for IPv6, motivated by the fact that its addresses are not merely more
numerous; they are different in kind. We introduce the notion of classifying
addresses and prefixes in two ways: (1) temporally, according to their
instances of activity to discern which addresses can be considered stable; (2)
spatially, according to the density or sparsity of aggregates in which active
addresses reside. We present measurement and classification results numerically
and visually that: provide details on IPv6 address use and structure in global
operation across the past year; establish the efficacy of our classification
methods; and demonstrate that such classification can clarify dimensions of the
Internet that otherwise appear quite blurred by current IPv6 addressing
practices.
