Each regional Internet Registry (RIR) has a whois page that allows you to enter an IP address (or subnet) and receive confirmation of the registered address assignee.
ARIN (North America) - http://whois.arin.net/ui
RIPE (Europe, Middle East, parts of Asia) - https://apps.db.ripe.net/search/query.html
APNIC (Asia Pacific) - http://wq.apnic.net/apnic-bin/whois.pl
...etc. for AFRNIC and LACNIC.
Here's a link to a list of country codes and associated registries:
http://www.ripe.net/internet-coordination/internet-governance/internet-technical-community/the-rir-system/list-of-country-codes-and-rirs
...and a map:
http://www.ripe.net/internet-coordination/internet-governance/internet-technical-community/the-rir-system