$TTL    86400
@       IN      SOA     ns1.example.com. admin.example.com. (
                            1         ; Serial
                            3600      ; Refresh
                            1800      ; Retry
                            604800    ; Expire
                            86400 )   ; Minimum TTL
        IN      NS      ns1.example.com.
ns1     IN      A       172.20.0.10

; Delegate subdomain to subdns
sub     IN      NS      ns1.sub.example.com.
ns1.sub IN      A       172.20.0.20