  This paper uses two commercial datasets of IP addresses from smartphones,
geolocated through the Global Positioning System (GPS), to characterize the
geography of IP address subnets from mobile and broadband ISPs. Datasets that
ge olocate IP addresses based on GPS offer superlative accuracy and precision
for IP geolocation and thus provide an unprecedented opportunity to understand
both the accuracy of existing geolocation databases as well as other properties
of IP addresses, such as mobility and churn. We focus our analysis on large
cities in the United States.
  After evaluating the accuracy of existing geolocation databases, we analyze
the circumstances under which IP geolocation databases may be more or less
accurate. We find that geolocation databases are more accurate on fixed-line
than mobile networks, that IP addresses on university networks can be more
accurately located than those from consumer or business networks, and that
often the paid versions of these databases are not significantly more accurate
than the free versions. We then characterize how quickly subnets associated
with fixed-line networks change geographic locations, and how long residential
broadband ISP subscribers retain individual IP addresses. We find, generally,
that most IP address assignments are stable over two months, although stability
does vary across ISPs. Finally, we evaluate the suitability of existing IP
geolocation databases for understanding Internet access and performance in
human populations within specific geographies and demographics. Although the
median accuracy of IP geolocation is better than 3 km in some contexts, we
conclude that relying on IP geolocation databases to understand Internet access
in densely populated regions such as cities is premature.
