Today's DNS system functions as a distributed database through a client-server relationship between DNS servers and clients requiring name resolution. The entire namespace of all domains comprises the DNS namespace. By using a distributed database architecture, DNS
provides for local control of each domain while still enabling all clients to access the entire database whenever needed.
The DNS namespace comprises a hierarchical structure of domains, with each domain representing a branch on the tree and subdomains residing underneath. At the topmost level are the root servers that maintain the root domains, such as .com, .net, .org, .biz, and so on. The root of the domain namespace is a null, often represented by a dot (.). Figure 17-1 illustrates the DNS namespace.
The root servers maintain only a limited amount of information about a given domain. Typically, the information includes only the name servers identified as authoritative for the zone (that is, those having authority over the domain's records). Records that map names to addresses within individual domains reside on the name server(s) for the domains in question. These name servers are typically managed by ISPs for the ISPs' clients or by companies that manage their own domains. Certain other domains are delegated to other organizations (ISPs, state agencies, educational institutions, and so on) that manage the domains for the respective domain holders. Distributing the DNS namespace in this way enables users to control their own domains while still remaining a part of the overall namespace.
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