This list summarizes the top 20 attacking class C (/24) subnets over the last three days.

Abstract from the maintainer's site:

The Internet Storm Center gathers millions of intrusion detection log entries every day, from sensors covering over 500,000 IP addresses in over 50 countries. It is rapidly expanding in a quest to do a better job of finding new storms faster, identifying the sites that are used for attacks, and providing authoritative data on the types of attacks that are being mounted against computers in various industries and regions around the globe. The Internet Storm Center is a free service to the Internet community. The work is supported by the SANS Institute from tuition paid by students attending SANS security education programs.

The ISC relies on an all-volunteer effort to detect problems, analyze the threat, and disseminate both technical as well as procedural information to the general public. Thousands of sensors that work with most firewalls, intrusion detection systems, home broadband devices, and nearly all operating systems are constantly collecting information about unwanted traffic arriving from the Internet. These devices feed the DShield database where human volunteers as well as machines pour through the data looking for abnormal trends and behavior. The resulting analysis is posted to the ISC's main web page where it can be automatically retrieved by simple scripts or can be viewed in near real time by any Internet user.

Dshield.org, provides also another interesting service to log contributors: the the Highly Predictive Blacklist.

They compare your firewall logs to firewall logs submitted by others. If you and other submitters are hit on similar ports, then your are more likely to be attacked by the same IPs. Your personal "HPB" is created from the IP addresses that target submitters with similar reports as you.