Freebsd Bind DNS server - multiple subnets

Hello all,

I have configured Freebsd 8.1 Bind DNS as DNS server for intrenal domain and clients on one subnet 192.168.10.0/24 . What do I have to change in zone file and in in-addr.arpa zone file if I have two additional subsets 192.168.20.0/24 and 192.168.30.0/24? Do I have to create another zone file for every subnet or I can just add additional subnets to existing files?

Thank you in advanced and kind regards,
Mark