DNS zone

In short: A DNS zone is a part of the DNS namespace managed by a name server, together with all records for it.

In more detail: The zone is the unit of administration: whoever looks after a zone decides which names exist in it and what they point to. It usually corresponds to a domain, such as example.com with all its records.

In Depth

A zone is stored in a zone file (or in the DNS provider’s interface):

example.com.   3600  SOA  ns1.example.com. admin.example.com. (...)
example.com.   3600  NS   ns1.example.com.
example.com.   3600  A    203.0.113.10
www            3600  CNAME example.com.
example.com.   3600  MX   10 mail.example.com.
example.com.   3600  TXT  "v=spf1 ..."

• SOA: starting values of the zone (responsible person, serial number).
• NS, A/AAAA, CNAME, MX, TXT: the records for servers, names, mail and extra info such as SPF.
• TTL: how long others may cache the answer.
• Zone and domain: a subdomain can sit in the same zone or be delegated as its own zone to other name servers.
• Zone transfer: the secondary name server copies the zone from the primary (only for permitted servers, otherwise an information leak).

See also: DNS server, nslookup