Every DNS database file begins with a Start of Authority (SOA) resource record. Whenever you alter any data in a DNS database file, you must increment the SOA serial number by one integer.
For example, if the current SOA Serial Number in a data file is 101, and you make a change to the file's data, you must change 101 to 102. If you fail to change the SOA serial number, the domain's secondary servers will not update their copy of the database files with the new information and the primary and secondary servers will become out of synch.
A typical SOA record of a sample hosts file looks like this:
; sample hosts file @ IN SOA nismaster.doc.com. root.nismaster.doc.com. ( 109 ; Serial 10800 ; Refresh 1800 ; Retry 3600000 ; Expire 86400 ) ; Minimum
Thus, if you made a change to this hosts file, you would change 109 to 110. The next time you change the file, you would change 110 to 111.