From http://whatismyipaddress.com/geolocation-accuracy:
"Determining the nation of an Internet user based on his or her IP address is relatively simple and accurate (95%-99% percent) ... "
"Determining the physical location down to a city or ZIP code, however, is more difficult and less accurate because there is no official source for the information ... "
"Accuracy rates on deriving a city from an IP address fluctuate between 50 and 80 percent, according to DNS Stuff, a Massachusetts-based DNS and networking tools firm".
Why? Because ISPs move IP address blocks around a lot.
