Sender Policy Framework

If delegation occurs, a new DNS Zone is formed. A zone starts at a particular domain and contains all of the partial tree below it, until another zone starts. This is best explained with an example: Zone "example.com." exists. The keeper of this zone has created the following domains:

 www.example.com.
 ftp.example.com.
 eu.example.com.
 us.example.com.
 mail.example.com.
 (in addition: domain example.com. also exists)

Then he delegates authority over "eu.example.com." and "us.example.com." elsewhere, meaning two new zones are formed.

 Zone "example.com." contains domains example.com. , www.example.com. , ftp.example.com. and mail.example.com.
 Zone "eu.example.com." contains domain "eu.example.com."
 Zone "us.example.com." contains domain "us.example.com."

Its respective keepers can create more domains in their own zone, for instance "www.eu.example.com." which is not part of zone "example.com." !

A new zone can be recognized because there will be a special Resource Record, a SOA record, defining authority over that part of the tree. There will also be NS records, pointing to servers serving this zone's data.