We offer a free database that maps IPv4 addresses to ASN, including the name of the ASN. The GeoIPASNum.dat.gz file works with our APIs, by calling the same methods as used by the organization database. The GeoIPASNum2.zip file contains a CSV formatted database. The database is updated every month.

Please visit the MaxMind Forum if you encounter problems or have any questions.

Team Cymru has a IP to ASN Lookup Page.