Is it safe to assume uniform geo-ip resolution for same first-16-bit IP addresses?

I have a geo-sensitive webapp for which I send a request's IP to a remote, commercial ip-to-location service, and get back the country, city, ISP, etc. for the IP.

I currently cache the IP lookups in my database in order to make subsequent lookups faster and free (the commercial service charges per lookup).

I wonder if I can further optimi开发者_开发知识库ze my caching by assuming that the first 16 bits (i.e. the aaa.bbb in a aaa.bbb.ccc.ddd addresss) always have a uniform location. That way I can have at most 2^15 records to cache. I don't mind so much about uniformity of ISP but that info would be helpful as well.

I'd recommend going down to at least /24 resolution. Oftentimes a /16 will tell you the ISP but not the city, or vice versa.

If you want a good idea of what the maps really look like, you can spend 49 USD on a developer license to Geobytes's GeoNetMap database. A developer license allows you to download the entire map from IP blocks to locations as a bunch of CSV files, but doesn't cover deploying it onto a production server. Geobytes has the added advantage of being entirely local, so lookups are liquid fast.

MaxMind also has a free downloadable map offering, although it is somewhat cut down from the full map, producing approximately double the error rate.

No, it's not safe. For example, if you do a GeoIP lookup on 216.34.181.45 (Slashdot) you get Mountain View, California. If you do a lookup on 216.34.1.1 you get Chesterfield, Missouri.

With respect to your caching, keep in mind that IPs can move around spatially. If an ISP goes bankrupt and its block gets bought by someone else, that block of IPs will move location.