ttl

This field is the default number of seconds to be used for the time-to-live field on resource records that don't have a ttl specified elsewhere.

There should only be one SOA record per zone. Example 28-3 is a sample SOA resource record.

Example 28-3 Sample SOA Resource Record

;name class 		SOA 	origin				 person-in-charge
doc.com. IN		SOA	dnsmaster.doc.com. root.nismaster.doc.com. (
							101			;Serial
							7200		;Refresh
							3600		;Retry
							432000		;Expire	
							86400)		;Minimum			 )