|Ah the confusion of information. That is a tricky question. What is the context of the question?|
I believe the ethernet they were talking about was a protocol like tcp/ip. Tcp/ip is 32bit whereas ethernet was 48bit.
If you do a define: ethernet in google you get a lot of specific definitions. You just need to figure out the right context to the definitions.
As a suggestion perhaps you could figure out how many addresses you would need for 15,000 hosts by figuring out the class and subnet that supported that number of hosts.
But then again if just talking about ethernet your math would be correct.