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Domain Name System

* Domain Name System (DNS) is a client-server application that identifies each host on the Internet with a unique user-friendly name.
* DNS organizes the name space in a hierarchical structure to decentralize the responsibilities involved in naming.
* DNS can be pictured as an inverted hierarchial tree structure with one root node at the top and a maximum of 128 levels.
* Each node in the tree has a domain name.
* A domain is defined as any subtree of te domain name space.
* The name space information is distributed among DNS servers. Each server has jurisdiction over its zone.
* A root server's zone is the entire DNS tree.
* A primary server creates, maintains, and updates information about its zone.
* A secondary server gets its information from a primary server.
* The domain name space in the Internet is divided into three sections: generic domains, country domains, and inverse domains.
* There are seven traditional generic labels, each specifying an organization type. Recently some new labels have been added.
* Each country domain specifies a country.
* The inverse domain finds a domain name for a given IP address. This is called address-to-name resolution.
* Name servers, computers that run the DNS server program, are organized in a hierarchy.
* The DNS client, called a resolver, maps a name to an address or an address to a name.
* In recursive resolution, the client may send its request to multiple servers before getting an answer.
* In iterative resolution, the client may send its request to multiple servers before getting an answer.
* A fully qualified doman name (FQDN) is a domain name consisting of labels beginning with the host and going back through each level to the root node.
* A partially qualified domain name (PQDN) is a domain name that does not include all the levels between the host and the root node.
* There are two types of DNS messages: queries and responses.
* There are two types of DNS records: question records and resource records.
* Dynamic DNS (DDNS) automatically updates the DNS master file.
* DNS uses the services of UDP for messages of less than 512 bytes; otherwise, TCP is used.