If your company is large enough, it may support a number of domains,organized into a local namespace. Figure 28-4 shows a domain hierarchy that might be in place in a single company. The top-level, or "root" domain for the organization is ajax.com, which has three sub-domains, sales.ajax.com, test.ajax.com, and manf.ajax.com.
DNS clients request service only from the servers that support their domain. If the domain's server does not have the information the client needs, it forwards the request to its parent server, which is the server in the next-higher domain in the hierarchy. If the request reaches the top-level server, the top-level server determines whether the domain is valid. If it is not valid, the server returns a "not found" type message to the client. If the domain is valid, the server routes the request down to the server that supports that domain.