Suppose a domain [dom.net] has the following: - one primary NS [PNS] - one secondary NS [SNS1] - one seconday NS [CNS2] loads from SNS1 - one caching-only NS [CNS] The SOA records for: PNS: 21600 ; refresh - 6 hours 3600 ; retry - 1 hour 2592000 ; expire - 1 month 345600 ) ; TTL - 4 days