What a zone holds
A zone file is a text collection of all the DNS records for a domain. Each entry describes one resource:
- SOA — the zone's start-of-authority record with refresh parameters and the primary name server.
- NS — the list of name servers responsible for the zone.
- A, AAAA — the addresses of hosts and services in the domain.
- MX and TXT — mail servers and free-form text data.
All of these records belong to the same domain and are edited by the zone administrator as one whole.
Zone delegation
The DNS namespace is hierarchical, so a single zone can be split into child zones. A parent zone such as example.com can delegate a subdomain — say shop.example.com — into a separate zone.
Delegation is done with NS records: the parent zone lists the name servers that are authoritative for the child zone. Responsibility for part of the tree passes to another administrator, and zone boundaries need not match domain boundaries.
