One of my servers started getting heavily loaded a few weeks ago for a few hours, so I did some studying and wrote a script to use netstat to get the IP addresses connected and the count. I put a new chain in iptables and if an IP is using more than 40 connections, it gets added to that chain which is then flushed every hour just to make sure no legitimate IP is blocked forever. If an IP is connected more than 100 times, it gets added directly to the INPUT chain and therefore is permanent until manually removed.

I'm mainly trying to figure out if those counts are good limits. Can there be legitimate reasons for an IP to be using more than 40 connections at a time? I tested going to a web page with 200 thumbnail images, and even then my IP was only listed a few times.