Track down Host Name to IPs

Greetings,

While monitoring our bandwidth the destination IP addresses frequently do not resolve to a host name, such as netflix, etc. Trying to make sure our users aren't abusing our bandwidth and obviously doing what they are paid to do. Tried many sites for lookups, trace routes, etc. Suggestions for tools to map IPs to domain names?

This is based on reverse dns, and if it is not configured by the users of the ip adresses, Ripe or IANA is your best option.