As discussed in various other threads, when a client accesses the global pool zone, the pool tries to determine the location of the client at a country granularity, and will then provide IP addresses of servers registered in the respective country zone to the client. In case the DNS system does not pass the client’s IP address to the pool’s authoritative servers (via the DNS EDNS Client Subnet (ECS) extension), the pool will instead use the IP address of the resolver contacting the autoritative server.
To better understand how a client’s IP address maps to a resolver (and vice-versa), and how well an IP address seen by a server matches with what was conveyed via ECS (if available), Ask is running the NTP Pool DNS Mapper. The goal is for that research to help improve the DNS system used by the pool.
In this context, there was an interesting talk at the currently ongoing RIPE 89 meeting that looked at how the resolver “market” evolved in the recent past, but more interestingly also analysed at a high level how clients of the big anycast resolvers are being served on a geographical basis, creating what the presenter calls the nicely illustrated titular “DNS watersheds”.
Both the slides as well as a recording of the talk are available online.