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Delegating zones

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Delegating zones

DNS provides the option of dividing up the namespace into one or more zones, which can then be stored, distributed, and replicated to other DNS servers. When deciding whether to divide your DNS namespace to make additional zones, consider the following reasons to use additional zones:

  • A need to delegate management of part of your DNS namespace to another location or department within your organization.

  • A need to divide one large zone into smaller zones for distributing traffic loads among multiple servers, improve DNS name resolution performance, or create a more fault-tolerant DNS environment.

  • A need to extend the namespace by adding numerous subdomains at once, such as to accommodate the opening of a new branch or site.

If, for any of these reasons, you could benefit from delegating zones, it might make sense to restructure your namespace by adding additional zones. When choosing how to structure zones, you should use a plan that reflects the structure of your organization.

When delegating zones within your namespace, be aware that for each new zone you create, you will need delegation records in other zones that point to the authoritative DNS servers for the new zone. This is necessary both to transfer authority and to provide correct referral to other DNS servers and clients of the new servers being made authoritative for the new zone.

When a standard primary zone is first created, it is stored as a text file containing all resource record information on a single DNS server. This server acts as the primary master for the zone. Zone information can be replicated to other DNS servers to improve fault tolerance and server performance.

When structuring your zones, there are several good reasons to use additional DNS servers for zone replication:

  1. Added DNS servers provide zone redundancy, enabling DNS names in the zone to be resolved for clients if a primary server for the zone stops responding.

  2. Added DNS servers can be placed so as to reduce DNS network traffic. For example, adding a DNS server to the opposing side of a low-speed WAN link can be useful in managing and reducing network traffic.

  3. Additional secondary servers can be used to reduce loads on a primary server for a zone.

Example: Delegating a subdomain to a new zone

As shown in the following figure, when a new zone for a subdomain ( is created, delegation from the parent zone ( is needed.

Example: How zone delegation works

In this example, an authoritative DNS server computer for the newly delegated subdomain is named based on a derivative subdomain included in the new zone ( To make this server known to others outside of the new delegated zone, two RRs are needed in the zone to complete delegation to the new zone.

These RRs include:

  • An NS RR to effect the delegation. This RR is used to advertise that the server named is an authoritative server for the delegated subdomain.

  • An A RR (also known as a glue record) is needed to resolve the name of the server specified in the NS RR to its IP address. The process of resolving the host name in this RR to the delegated DNS server in the NS RR is sometimes referred to as glue chasing.


  • When zone delegations are correctly configured, normal zone referral behavior can sometimes be circumvented if you are using forwarders in your DNS server configuration. For more information, see Directing queries through forwarders.