A zone is a contiguous portion of the DNS namespace. It contains a series of records stored on a DNS server. Each zone is anchored at a specific domain node. However, zones are not domains. A DNS domain is a branch of the namespace, whereas a zone is a portion of the DNS namespace generally stored in a file, and can contain multiple domains. A domain can be subdivided into several partitions, and each partition, or zone, can be controlled by a separate DNS server. Using the zone, the DNS server answers queries about hosts in its zone, and is authoritative for that zone. Zones can be primary or secondary. A primary zone is the copy of the zone to which the updates are made, whereas a secondary zone is a copy of the zone that is replicated from a master server.
Zones can be stored in different ways. For example, they can be stored as zone files. On Windows 2000 servers, they can also be stored in the Active Directory ™ directory service . Some secondary servers store them in memory and perform a zone transfer whenever they are restarted.
Figure 5.3 shows an example of a DNS domain that contains two primary zones. In this example, the domain reskit.com contains two subdomains: noam.reskit.com. and eu.reskit.com. Authority for the noam.reskit.com. subdomain has been delegated to the server noamdc1.noam.reskit.com. Thus, as Figure 5.3 shows, one server, noamdc1.noam.reskit.com, hosts the noam.reskit.com zone, and a second server, reskitdc1.reskit.com, hosts the reskit.com zone that includes the eu.reskit.com subdomain.
Figure 5.3 Domains and Zones
Rather than delegating the noam.reskit.com zone to noamdc1.noam.reskit.com, the administrator can also configure reskitdc1 to host the zone for noam.reskit.com.
Also, you cannot configure two different servers to manage the same primary zones; only one server can manage the primary zone for each DNS domain. There is one exception: multiple computers can manage Windows 2000 Active Directory–integrated zones. For more information, see "Windows 2000 DNS" in this book.
You can configure a single DNS server to manage one zone or multiple zones, depending on your needs. You can create multiple zones to distribute administrative tasks to different groups and to provide efficient data distribution. You can also store the same zone on multiple servers to provide load balancing and fault tolerance.
For information about what zones contain, see "Resource Records and Zones" later in this chapter.