reverse lookup entry in BIND configuration

[query]

Hi All,
I am just learning how to configure a DNS name server using BIND.

I am confused regarding the reverse lookup entry in /etc/named.conf

For reverse lookup the entry is something like

zone "16.16.172.in-addr.arpa" IN {
type master;
file "example.in.local";
allow-update { none; };
};

My confusion is regarding the 16.16.172.in-addr.arpa . Is it
necessary that for 16.16.172.in-addr.arpa , it should always
contain the first three octets in an ip address irrespective of
the class of ip address . Or it depends on ip address class. That
is for a B class address, the entry will be somewhat like

zone "16.172.in-addr.arpa" IN {
type master;
file "example.in.local";
allow-update { none; };
};

Also , it will be helpful to me if somebody can tell regarding the
significance of using half part of the ip address instead of using
the complete ip address.
Thanks in Advance