You can look up the provider who owns the IP block at
http://www.iana.org/whoisBe aware that the company owning the IP block is not neccessarily the ISP of your user. Some old but small ISPs rent out their IP space to other ISPs (i.e. you might find that the IP block is owned by "Small ISP from Africa or Latin America", but the user is actually from the US and customer of "Some big US ISP who rented IPs from Small ISP from Africa or Latin America") and some large companies have different subsidiaries for registering IP addresses and providing services to end users. (i.e. you will find the IP belongs to "Big Company IP Registration Service", but the User's ISP is actually "one out of 100 brand names of big company")