Location detecting techniques for IP addresses

If you looking for some copy - paste solution, i foudn this code :

    function countryCityFromIP($ipAddr)
{
//function to find country and city from IP address
//Developed by Roshan Bhattarai http://roshanbh.com.np

//verify the IP address for the
ip2long($ipAddr)== -1 || ip2long($ipAddr) === false ? trigger_error("Invalid IP", E_USER_ERROR) : "";
$ipDetail=array(); //initialize a blank array

//get the XML result from hostip.info
$xml = file_get_contents("http://api.hostip.info/?ip=".$ipAddr);

//get the city name inside the node <gml:name> and </gml:name>
preg_match("@<Hostip>(\s)*<gml:name>(.*?)</gml:name>@si",$xml,$match);

//assing the city name to the array
$ipDetail['city']=$match[2]; 

//get the country name inside the node <countryName> and </countryName>
preg_match("@<countryName>(.*?)</countryName>@si",$xml,$matches);

//assign the country name to the $ipDetail array
$ipDetail['country']=$matches[1];

//get the country name inside the node <countryName> and </countryName>
preg_match("@<countryAbbrev>(.*?)</countryAbbrev>@si",$xml,$cc_match);
$ipDetail['country_code']=$cc_match[1]; //assing the country code to array

//return the array containing city, country and country code
return $ipDetail;

}

Hope this helps somebody.

You can't do this without a database that maps IP addresses to cities/countries/providers. There are commercial offerings such as ip2location that you could use. AFAIK there is no free alternative though, as maintaining such a IP database is quite a lot of work. Free alternative: GeoIP2

Update: There are several things that allow you to create such a db, if you invest enough time:

1. Use the databases provided by regional and local registries to find an IP's owner.
2. Many ISPs use a naming schema that allows you to locate the user. Sometimes you can even read the city name in plain text if you do a reverse-DNS lookup. Sometimes it is more cryptic. For example I currently have p5dcf6c4a.dip.t-dialin.net , and I have no idea that the naming scheme is..
3. Use a traceroute. If you can't identify the location of a user, you can still find out the location of its uplink