This database provides a reference to determine the autonomous system and number (ASN). An autonomous system (AS) is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators as a single administrative entity in routing policy.

It is free for personal or commercial use with attribution required by mentioning the use of this data.

Downloadable CSV file. Update cycle unknown.

A discussion of reserved netblocks (i.e., RFC 1918 and others which are set aside for specific purposes and as such are not advised for public use).

This web app generates text files of netblocks in CIDR format, suitable for dropping into firewalls to deny traffic from just about any country on the planet.