1.5 Zone

We often come across the questions: What is a zone? What is the relation between a domain and a zone? Let us explain the relationship of these terms using the company.com domain.

As we have already said, a domain is a group of computers that share a common right side of their domain name. For example, a domain is a group of computers whose names end with company.com. However, the domain company.com is large. It is further divided into the subdomains bill.company.com, sec.company.com, sales.company.com, xyz.company.com, etc. We can administer the entire company.com domain on one name server, or we can create independent name servers for some subdomains. (In Figure 1.3, we have created subordinate name servers for the subdomains bill.company.com and head.company.com.) The original name server serves the domain company.com and the subdomains sec.company.com, sales.company.com, and xyz.company.com—in other words, the original name server administers the company.com zone. The zone is a part of the domain namespace that is administered by a particular name server.

Figure 1.3: Zone company.com

A zone containing data of a lower-level domain is usually called a subordinate zone.

1.5.1 Special Zones

Besides classic zones, which contain data about parts of the domains or subdomains, special zones are also used for DNS implementation. Specifically, the following zones are used:

• Zone stub: Zone stub is actually a subordinate zone that only contains information about what name servers administer in a particular subdomain (they contain the NS records for the zone). The zone stub therefore does not contain the entire zone.
• Zone cache/hint: A zone hint contains a list of root name servers (non-authoritative data read into memory during the start of the name server). Only BIND version 8 and later use the name hint for this type of zone. In previous versions, a name cache zone was used. Remember that the root name servers are an authority for a root domain marked as a dot (.).