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.
How CGNAT affects IP geolocation for mobile users
A single public IP address on a mobile network can, at any given moment, represent a genuinely large number of distinct devices, all routed through the same carrier gateway under carrier-grade NAT. This is not an occasional occurrence on certain mobile networks, it is the standard operating model for how most carriers manage a limited pool of public IPv4 addresses across a much larger number of connected subscribers. Understanding this pooling is the key to understanding what mobile IP geolocation can and cannot promise.
Because so many distinct subscribers can share one visible public address at once, that address cannot, by definition, point to any one subscriber's precise location. What it can reasonably identify is the carrier itself and, depending on how that carrier structures its network, the general region the gateway serves. Some carriers route NAT pools through a small number of centralized exit points covering wide areas, which pushes mobile geolocation toward country or broad-region accuracy. Others maintain more distributed gateways closer to where subscribers actually are, which allows city-level results to hold up meaningfully better.
This variation means mobile IP accuracy is not a single fixed ceiling across the board, it depends on how a given carrier has architected its own network, which is outside any geolocation provider's control. A city-level result for a mobile IP is a genuinely useful estimate in many cases, but it should be understood as probabilistic, reflecting where that carrier's infrastructure tends to route traffic from a broader area, rather than a confirmed pinpoint of the device.
The practical response is to calibrate expectations to the connection type. If your organization and asn fields in an IP lookup identify a known mobile carrier, treat any city-level result with appropriately wider tolerance than you would for a fixed residential or business broadband connection, where NAT pooling is far less aggressive and location tends to track more tightly to the subscriber's actual area. For use cases like fraud scoring or regional content delivery, this distinction between connection types, not just the raw location itself, is often the more actionable signal.
My Geocode's IPv4 and IPv6 lookups return ASN and organization alongside geographic fields specifically so you can identify when a request is coming from a mobile carrier network and adjust your accuracy expectations accordingly, rather than treating every IP-derived location as equally precise regardless of connection type.