Right, your webserver is not a FreeBSD/OpenBSD system? Which OS are you using? You may want to use that system's firewall.
Adding a new FreeBSD or OpenBSD router would also be possible, and the effects would be the same except that you would draw more power
As mentioned above, pf ("The OpenBSD Packet Filter", also available on FreeBSD) has several options (
overload,
max-src-nodes,
max-src-conn,
max-src-conn-rate) which can be used to limit the maximum amount connections one host can make.
The documentation mentioned before has more specific documentation as well as some examples.