I wrote a blog post a while back explaining why NetFlow v5 isn't a good way of doing web usage tracking:
There are many flow exporter products now available that use IPFIX to export HTTP host header data to a flow collector, but NTA doesn't yet support any of these custom record types. In the Cisco product line, you'd need an ISR G2 or ASR 1K router with very recent code and a "data" license to get these exports.
There's at least one 3rd party product that integrates with NPM that looks at a SPAN session and extracts HTTP data and makes it available in NPM dashboards.
Reason is simple:
PING hulu.com [126.96.36.199]
C:\Users\Me>ping -a 188.8.131.52
PING a23-63-99-184.deploy.static.akamaitechnologies.com [184.108.40.206]
The hulu.com is hosted on akamai servers, so reverse DNS lookup returns akamaitechnologies.com domain instead of hulu.com. NTA obtains hostnames/domains using reverse DNS lookup as NetFlow only provides IP addresses.
(netflix.com reverse to netflixfreetrial.com)
Just to answer also the question from thread subject - "Please enter a valid domain" means that either there are no data for such domain or entered domain is invalid.