This host can be anywhere in the world. There isn't a central authority that you can contact for these cases, unfortunately. The best way is to contact the hosts ISP and report an abuse.
The way to find what is the abuse address for a certain IP is to use whois. There is a whois utility on Linux and on MacOS X. If you don't have this on your machine, try a whois website:
Notice how the whois entry shows very clearly what is the ABUSE e-mail address. This is not always so clear, but it's often there. If not, you can try using abuse@ISP, where ISP is the domain.
After you get this address, send a very polite and informative e-mail to this address. Add all data that you have - what was the attack you received, includ the log entries and relevant date/time. Mention the source IP and the destination IP (the one on your side).
Last but not least, a good resource for abuse reports is this website:
This is not the please where you will fill your complain, but they will help you find where to send your abuse report, and how do to it effectively.