DNS Bind SOA file

This article is about the SOA files which is referenced from zone file described at DNS Bind zone file configuration - Shammerism's article.
SOA file should have some elements. Here is a sample of the SOA of examples.xyz, defined as /etc/bind/db.examples.xyz in DNS Bind zone file configuration - Shammerism.

$TTL	86400 
@       IN      SOA     dns.examples.xyz root.examples.xyz. ( 
                        2               ; Serial 
                        604800          ; Refresh 
                        86400           ; Retry 
                        2419200        	; Expire 
                        604800 )        ; Negative Cache TTL 
; 
        IN      NS      dns.examples.xyz. 
dns     IN      A       X.X.X.X 
www  IN      A       X.X.X.X  
...

TTL means how long cached this record information in cache DNS servers. SOA stands for Start Of Authority and @IT Description is good about the parameters.

And SOA file should be prepared not only for normal lookup but also reverse lookup. Here is a sample of reverse lookup defined as /etc/bind/db.192.168.1 in DNS Bind zone file configuration - Shammerism.

$TTL    604800 
@       IN      SOA     dns.examples.xyz. root.examples.xyz. ( 
                              1         ; Serial 
                         604800         ; Refresh 
                          86400         ; Retry 
                        2419200         ; Expire 
                         604800 )       ; Negative Cache TTL 
; 
@       IN      NS      dns.examples.xyz. 
X      IN      PTR     www.examples.xyz. 
...

If this dns server received a query for www.examples.xyz, this server returns 192.168.1.X as a response.