For IPv4: Maxmind (or Google data if you can get that ;) For IPv6: RIR data (which is what GRH uses, which is what is used by the little flag at the bottom of this page) The tricks employed by Maxmind and other such companies is basically collecting/buying lots of IP,city,country combinations from sites like Ebay and other places where you fill in your details. They assume that the address you enter is the address you are at that moment, then, based on how often a combination occurs, take the best match. For IPv6 there is simply too little use at the moment to do the above trick, also when they start doing it, it will most likely get limited per /48, as that should be a single site at a single location.