@ IN NS dns.bob.org. $ORIGIN 2.1.IN-ADDR.ARPA. 3 IN SOA bob.org. root.bob.org. ( 2001121401 ; serial 10800 ; refresh 3600 ; retry 604800 ; expire 86400 ; default_ttl ) 3 IN NS dns.bob.org. 3 IN NS dns2.bob.org. $ORIGIN 3.2.1.IN-ADDR.ARPA. 0 IN PTR network.bob.org. 1 IN PTR gateway.bob.org. 2 IN PTR dns.bob.org. 3 IN PTR dns2.bob.org. 4 IN PTR bob.org. 5 IN PTR bob5.bob.org. 6 IN PTR bob6.bob.org. [Skipped a few lines] 253 IN PTR bob253.bob.org. 254 IN PTR bob254.bob.org. 255 IN PTR broadcast.bob.org.
Whenever you change any of these two files you will need to also update the serial number. Usually the serial number is just another form of the date so that you always create later numbers. In this example the serial number is the date in the form YYYYMMDD## where ## stands for the version number for the day. It is quite possible to change the serial number for the DNS more than 10 times a day so it is best to have 2 digits to use. Also, whenever you make a change you will need to restart the DNS in order to have the changes take affect.
bash# /etc/rc.d/init.d/named restart