区域文件解释，DNS？

我有一个问题，从wikipedia.com这个区域文件“

$ORIGIN example.com. ; designates the start of this zone file in the namespace $TTL 1h ; default expiration time of all resource records without their own TTL value example.com. IN SOA ns.example.com. username.example.com. ( 2007120710 ; serial number of this zone file 1d ; slave refresh (1 day) 2h ; slave retry time in case of a problem (2 hours) 4w ; slave expiration time (4 weeks) 1h ; maximum caching time in case of failed lookups (1 hour) ) example.com. NS ns ; ns.example.com is a nameserver for example.com example.com. NS ns.somewhere.example. ; ns.somewhere.example is a backup nameserver for example.com example.com. MX 10 mail.example.com. ; mail.example.com is the mailserver for example.com @ MX 20 mail2.example.com. ; equivalent to above line, "@" represents zone origin @ MX 50 mail3 ; equivalent to above line, but using a relative host name example.com. A 192.0.2.1 ; IPv4 address for example.com AAAA 2001:db8:10::1 ; IPv6 address for example.com ns A 192.0.2.2 ; IPv4 address for ns.example.com AAAA 2001:db8:10::2 ; IPv6 address for ns.example.com www CNAME example.com. ; www.example.com is an alias for example.com wwwtest CNAME www ; wwwtest.example.com is another alias for www.example.com mail A 192.0.2.3 ; IPv4 address for mail.example.com, ; any MX record host must be an address record ; as explained in RFC 2181 (section 10.3) mail2 A 192.0.2.4 ; IPv4 address for mail2.example.com mail3 A 192.0.2.5 ; IPv4 address for mail3.example.com " Why is these lines necessary?: example.com. NS ns ; ns.example.com is a nameserver for example.com ns A 192.0.2.2 ; IPv4 address for ns.example.com 

我们已经知道ns.example.com是默认的名称服务器，对吧？ 那么我们为什么要指定它的IP地址呢？ 在ns.example.com名称服务器中是不是这个区域文件，我的意思是，如果我们正在查看它，为什么我们要的服务器的IP地址，我们已经看到的文件？