Out of zone data BIND problem

Your origin for the zone is . per your configuration. You are creating records for ns1. and ns2. instead of ns1.example.com. and ns2.example.com. Since ns1.example.com and ns2.example.com aren't defined, they are matched by the wildcard.

EDIT: here's an edit of your config and zone:

zone "example.com." {
        type master;
        file "ext.zone";
};

ext.zone:

$TTL    3600
@       IN      SOA     ns1 root (
                              1         ; Serial
                         3600         ; Refresh
                          300         ; Retry
                         3600         ; Expire
                         300 )        ; Negative Cache TTL


        IN      NS      ns1
        IN      NS      ns2
        IN      A       192.0.2.6


ns1     IN      A       192.0.2.4
ns2     IN      A       192.0.2.5

*      IN      A       192.0.2.6

Everything in the zone is relative to the zone name in the named configuration, so adding a second zone just points to the same file:

zone "example.net." {
    type master;
    file "ext.zone";
};

Here is an example zone file for a domain that also hosts its own name server. The primary name server and mail server are on the same IP as the domain. Another machine hosts the secondary name server and the backup mail server. This other machine must be configured for both tasks, and you need glue records at the registrar.

$ttl 10800
example.com.    IN  SOA ns1.example.com. root.example.com. (
            1239719044
            3600
            3600
            604800
            38400 )
example.com.    IN  NS  ns1.example.com.
example.com.    IN  NS  ns2.example.com.
example.com.    IN  A   123.123.123.123
example.com.    IN  MX  0 mail.example.com.
example.com.    IN  MX  10 mail2.example.com.
example.com.    IN  TXT "v=spf1 a mx ptr ?all"
ns1.example.com.    IN  A   123.123.123.123
ns2.example.com.    IN  A   234.234.234.234
localhost.example.com.  IN  A   127.0.0.1
mail.example.com.   IN  A   123.123.123.123
mail2.example.com.  IN  A   234.234.234.234