Some IP blocks occasionally change their physical location, such as when blocks are transferred to different organizations, or repurposed within an organization. IP geolocation systems need to identify such changes to provide accurate results for location-dependent applications such as geo-blocking and online fraud prevention. We propose an efficient method to identify IP blocks that move, since full geolocation is expensive and unnecessary for blocks that do not move. Our approach uses persistent changes in latency as an indicator of block movement, tracking all ping-responsive IPv4 /24 blocks from a handful of globally distributed vantage points. We estimate around 2.1% of the 3.77M /24 blocks we studied have changed location at least once in the last 3 months of 2018. We find that the remaining blocks were consistently RTT-stable during the same period, suggesting that their locations were also stable. We validate a random sample of blocks we identify as moving and confirm 80% (41 of 51) through traceroutes.