How to properly use sub-domain

[John Sherling]

I'm running BINC 9.3.2, and I have a zonefile on my BIND server called
myDomain.com. I want to set up a separate sub-domain, called
'sub.myDomain.com' and have the data server from a different zone
file, but on the same server.

But, my question is, how would my DNS server know how to differentiate
between a request for 'host.myDomain.com' and 'host.sub.myDomain.com'
and look in the appropriate zone file for each?

Many thanks...

J