$ cat /etc/bind/named.conf.local
include "/etc/bind/zones.rfc1918";
zone "example.com" {
    type master;
    file "/etc/bind/zones/db.example.com";
    allow-transfer { any; };
    update-policy local;
};

$ cat /etc/bind/zones/db.example.com
$ORIGIN .
$TTL 172800     ; 2 days
example.com               IN SOA  ns1.example.com. hostmaster.example.com. (
                          116        ; serial
                          7200       ; refresh (2 hours)
                          900        ; retry (15 minutes)
                          1857600    ; expire (3 weeks 12 hours)
                          8400       ; minimum (2 hours 20 minutes)
                          )
                    NS      ns1.example.com.
                    NS      ns1.external-host.com.
                    NS      ns2.external-host.com.
ns1.external-host.com      A       1.2.3.1
ns2.external-host.com      A       1.2.3.2

$ORIGIN example.com.
ns1                     A       1.1.1.1
@            IN         NS      ns1.external-host.com.
@            IN         NS      ns2.external-host.com.
www            IN         NS      ns1.external-host.com.
www           IN         NS      ns2.external-host.com.
clients                 A       1.1.1.1
$ORIGIN clients.example.com.
$TTL 3600       ; 1 hour
test                    A       2.2.2.2