I have a Windows 2008R2 Server running IIS 7.5. It is getting bombarded with attempted hacks from similar IPs.
How would I block the IP range with something like 51.255.65.*?
Below is the UI presented to me when adding a Deny restriction
You need to specify the range in proper subnet format. You may wish to take a look at our canonical question on IPv4 subnetting, if you're not exactly sure what that means.
For your example, you'd use
220.127.116.11 in the "IP address range" field, and the subnet mask of
255.255.255.0. in the "Mask or Prefix" field.