Authority for subdomains

[The Tanster]

How can one set up a Master DNS server for a subdomain that one is
running on a private network to be authoritative for that subdomain only
(i.e. have a nslookup or dig query return an "authoritative" instead of
a "non-authoritative" answer when looking up a host only within that
subdomain)?  Can it be done in the named.conf through some keyword?
I've looked through the O'Reilly DNS & BIND book by Cricket Liu et al
and I can't find any information on this.  Thanks in advance.