A. Network, Application & User Monitoring
B. Network Planning
C. Security Analysis
D. Accounting /Billing
My guess is A, B, C
not c, so all others
Unfortunately that's an example of a poorly worded question...because technically all four answers are correct. But if I had to choose only three, I'd select A, B, and C.
Netflow can be used for Security Analysis because you can spot trends in your network of network flows that should NOT be there. For example, if you know that certain attack vectors use UDP port 3456 (I just made that up) and you see from Netflow a lot of flows to/from UDP port 3456 now you know that an attack is going on. Or if your previous historical analysis has revealed (through Netflow) that 90% of the traffic coming/going to your network is HTTPs and only 10% is regular, unsecured HTTP...but suddenly Netflow reveals that HTTP has increased to 50% of your traffic...that might indicate a security issue.
Similarly, answer D is also correct because with flow-based information you can create accounting and billing records. The only reason why D is "less correct" than the other questions is because most clients have laptops, PCs, tablets, etc that utilize DHCP for IP addressing. As such, billing-and-accounting is hard to do based on an IP address of a packet, because you never know what IP address has been allocated via DHCP to a given client at any given time. Billing-and-accounting is much easier to implement based on user-credentials (like passwords) rather than IP addresses.
Cisco IOS NetFlow efficiently provides a key set of services for IP applications, including network traffic accounting, usage-based network billing, network planning, security, Denial of Service monitoring capabilities, and network monitoring. NetFlow provides valuable information about network users and applications, peak usage times, and traffic routing. Cisco invented NetFlow and is the leader in IP traffic flow technology.
hmmmm, looks like all 4 are ok