Another one of the great built-in features of IIS 8 is Dynamic IP Restrictions (DIPR). With a few simple configuration steps you can quickly set limits for blocking IP addresses based on the number of concurrent requests or frequency of requests over a period time. With these parameters in place IIS will take over blocking requests unattended thereby making your server more secure.
Source : Preventing Automated Attacks with IIS Dynamic IP Restrictions | Peter Viola
