Serial numbers are used by slave nameservers to determine when a zone transfer is necessary.
Bind9 not updating zone serial
The syntax of zone data files lends itself to making mistakes.
It doesn't help that the address and pointer records are in different files, which must agree with each other.
While on its face this may seem an excessively friendly default, DNS data is essentially public (that's why its there) and the bad guys can get all of it anyway.
However if the thought of anyone being able to transfer your precious zone file is repugnant, or (and this is far more significant) you are concerned about possible Do S attack initiated by XFER requests, then use the following policy.
We'll go through the steps here in case you weren't the one to set up those files or if you'd just like a checklist to follow.