ow do I restrict the number of connections used by a single IP address to my server for port 80 and 25 using iptables?
You need to use the connlimit modules which allows you to restrict the number of parallel TCP connections to a server per client IP address (or address block).
This is useful to protect your server or vps box against flooding, spamming or content scraping.
In the beginning was the firewall, and it was pretty good. A big box of rules that sat between your network and the evils of the Internet, the firewall examined ports and protocols to decide which packets got in and which were barred at the door. Then things got, as things often do, complicated. New threats came sneaking in on trusted protocols, ports and protocols became tangled, and looking inside packets became just as important as noting their source, destination, and type.