Basic Internet Architecture .com .edu .org .au .bigcorp.com
Basic Internet Architecture .com .edu .org .au .bigcorp.com .accounting .bigcorp.com .iana.org www.iana.org mail.accounting .bigcorp.com .com.au .edu.au Top-level domains Figure 4.18 Hierarchy within domain name structure reflects a hierarchy of delegation authority 4.3.4.2 DNS Hierarchy The hierarchy in a domain name essentially describes a search path across the distributed collection of name servers that together make up the Internet s DNS. Name servers are queried whenever a domain name needs to be resolved to an IP address. Certain name servers are responsible for being authoritative sources of information for particular domains or subdomains. The name servers ultimately responsible for each TLD are known as root name servers. Before hosts can use the DNS they must be configured with the IP address of a local name server the host s entry point into the DNS. The ISP or whoever supports your network typically provides the local name server. The name server s IP address is either manually configured into each host, or can be automatically configured (for example, DHCP provides an option for configuring the local name server s address [RFC2132]). Local name servers are manually configured to know the IP address of at least one root name server, and possibly another name server further up the domain name tree. Name servers either answer queries with local knowledge, or seek out another name server who is responsible for mappings higher up the domain name hierarchy. Local knowledge is often held in a cache built from recent queries from other hosts the cache allows rapid answers for frequently resolved domain names. For the curious reader: Many recent versions of Windows, and Unix-link operating systems such as Linux and FreeBSD have a tool called nslookup. Often installed as a command line application, nslookup allows you to manually perform DNS queries and explore your local network s DNS configuration. Similar tools may be found under names like dig or host. References [ISO3166] http://www.iso.org/iso/en/prods-services/iso3166ma/02iso-3166-code-lists/list-en1-semic.txt. [RFC768] J. Postel, Ed, User Datagram Protocol , RFC 768. August 1980. [RFC791] J. Postel, Ed, Internet Protocol Darpa Internet Program Protocol Specification , RFC 791. September 1981.
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