Testing location data with edge cases, not just happy paths
An address in a well mapped city center tells you almost nothing about how your system handles a rural route, a disputed border, or a query near the poles. Test the hard cases deliberately.
Anycast and CDN IP ranges: a special case for geolocation
Most IP geolocation assumes, reasonably enough, that a given address corresponds to a roughly fixed physical location or a stable, identifiable network. Anycast addressing breaks that assumption in a specific and interesting way, and it is worth understanding if you ever geolocate traffic coming from a content delivery network or a large-scale distributed service rather than an ordinary end-user connection.
With anycast, the identical IP address is announced from multiple physical locations at once, and internet routing delivers any given request to whichever announcing location is closest or best positioned from the requester's point of view at that moment. This is precisely how many content delivery networks achieve low latency at global scale, by having the same address respond locally to users everywhere rather than routing every request back to one central server. The practical consequence for geolocation is that the identical IP address can genuinely correspond to different physical server locations depending on who is asking and from where, which is a fundamentally different situation from an ordinary IP tied to one fixed location.
This means a geolocation lookup on an anycast address is best understood as describing the operator running that anycast network, and sometimes a representative or commonly used point within its distributed footprint, rather than a single fixed physical location the way a lookup on a typical residential or business IP would be. Treating the returned location as "exactly where the traffic is physically being served from" is a reasonable approximation for some purposes and can be meaningfully wrong for others, depending on exactly how that network's anycast routing behaves for a given request.
This matters most for anyone trying to geolocate the origin of traffic through, or from, a CDN or similar large distributed service. If you are trying to identify a website's visitor by IP but that visitor's request passed through a CDN edge server before reaching your logging or application layer, and you geolocate the CDN edge's IP rather than the original visitor's, you are geolocating the CDN's infrastructure, not the person you actually care about. Getting the real visitor's IP into your logs correctly, typically via a forwarded-for header set up properly at the CDN layer, matters far more here than any refinement to the geolocation service itself, since no geolocation lookup can recover a location the request layer never captured in the first place.
The asn and organization fields are the clearest signal that you are looking at CDN or anycast infrastructure rather than an ordinary end-user connection, and it is worth checking them specifically whenever geolocation results for a known service or platform look unexpectedly clustered around a small number of locations regardless of where its actual users are. Our IPv4 and IPv6 lookups return these fields precisely to support that kind of diagnosis.