IP addresses are commonly used to identify hosts or properties of hosts. The address assigned to a host may change, however, and the extent to which these changes occur in time as well as in the address space is currently unknown, especially in IPv6. In this work, we take a first step towards understanding the dynamics of IPv6 address assignments in various networks around the world and how they relate to IPv4 dynamics. We present inegrained observations of dynamics using data collected from over 3,000 RIPE Atlas probes in dual-stack networks. RIPE Atlas probes in these networks report both their IPv4 and their IPv6 address, allowing us to track changes over time and in the address space. To corroborate and extend our findings, we also use a dataset containing 32.7 billion IPv4 and IPv6 address associations observed by a major CDN. Our investigation of temporal dynamics with these datasets shows that IPv6 assignments have longer durations than IPv4 assignments—often remaining stable for months—thereby allowing the possibility of long-term fingerprinting of IPv6 subscribers. Our analysis of spatial dynamics reveals IPv6 addressassignment patterns that shed light on the size of the address pools network operators use in domestic networks, and provides preliminary results on the size of the prefixes delegated to home networks. Our observations can benefit many applications, including host reputation systems, active probing methods, and mechanisms for privacy preservation.