Dns (Domain Name System)
The Domain Name System (DNS) is the addressing and communications system by which computers connected to the Internet locate each other. Every time an Internet-connected user types in a domain name address, for example www.galegroup.com, the DNS translates this information into a numeric format that can be used to locate other computers on the Internet. This addressing system requires that each computer connected to the Internet have a unique numeric address or Internet Protocol (IP) address. As with most addressing systems (e.g., postal), it is necessary that each IP address be unique.
When an Internet user types in a domain name address, a DNS server (also called a name server; a computer acting as a switching node or post office for the rest of the network) takes that address and translates it into the appropriate numeric IP address. Machines easily recognize numeric IP addresses, such as 12.98.178.9, while people easily remember familiar word-type domain names such as www.galegroup.com. Because the Internet is based on IP addresses, not domain names, Internet protocol requires DNS servers to translate domain names into IP addresses, which are stored in databases maintained by the servers.
An IP address identifies a host computer connected to a Transmission Control Protocol/Internet Protocol (TCP/IP) network. TCP/IP is the suite of communications protocols used to interconnect hosts on the Internet. Routing of messages on the TCP/IP network is based on the IP address of the destination. IP addresses are 32-bit addresses that are expressed as four numbers, each from 0 to 255, separated by implicit periods. (That is, the periods are not coded explicitly in the address, but are used for visual clarity when the number is written out.) The first three numbers of the IP address 127.18.169.3 identify the network that the host is connected to; the last number of the IP address (in this case 3) identifies the host itself. The 32-bit IP address can identify almost 4 billion distinct hosts.
A domain name is used in the Universal Resource Locator (URL) to identify particular Web pages. A domain name is written in lowercase letters with words separated by periods, and takes the generic form www.host.zone. DNS uses a hierarchical structure to organize such addresses. At the top of the hierarchy is the root, technically called the dot. Beneath the root level are the top-level domains, called zones, which refer to the type of organization being addressed. Common zone names include .com (commercial businesses), .net (network organizations), .org (nonprofit organizations), .edu (educational institutions), .mil (the military), and .gov (government agencies). Beneath each of these top-level domains are second-level domains or hosts. The host, such as "microsoft" in "microsoft.com," is the name of an Internet service provider (ISP), an organization which provides access for a number of users' computers to the Internet. The host name may consist of several parts. A fictitious BCD Company might create a subdomain called "software.bcd.com" to represent the software division of its organization. A domain name represents a specific and unique portion of the domain name space, which is defined as the total number of computers and resources that are described and located through the DNS.
The DNS is, in fact, its own network. If one DNS server is not able to translate a particular domain name, it asks another server, and so on, until the correct IP address is returned. Specifically, when visiting a Web site on the Internet, a piece of software called the DNS resolver (usually built into the network operating system) sees to it that the correct IP address is delivered back to the host.
The DNS is, in effect, a distributed database containing information about domains and host computers within those domains. This distributed database is split up among computers dispersed across the Internet. Each of these computers operates as a DNS server (or name server). Each name server has authority over a particular piece of the database called its zone. However, the entire Internet can access the information possessed by any name server. This ability to exchange information is crucial to the functioning of the DNS. This distributed database is responsible for maintaining an enormous collection of data on all of the domains in the world; there is no centralized, complete record of this information.
Since the information used by the DNS is distributed among many DNS servers, it is extremely important that these servers run efficiently and maintain up-to-date information. The actual data that define a name server's area of authority are known as zone files.
If a server experiences a problem, it is likely that the rest of the network will also experience problems. The DNS uses several techniques to prevent problems from spreading from one machine to the rest of the network, but the interdependent nature of the system still requires careful management of the machines, software, and data that comprise the DNS.
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