The Domain Name System

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The Domain Name System

A name that identifies one or more IP addresses. For example, the domain name microsoft.com represents about a dozen IP addresses. Domain names are used inURL's to identify particular Web pages. For example, in the URL http://www.hotmail.com/ the domain name is hotmail.com.

Every domain name has a suffix that indicates which top level domain (TLD) it belongs

to. There are only a limited number of such domains. For example:

· gov - Government agencies

· edu - Educational institutions

· org - Organizations (nonprofit)

· mil - Military

· com - commercial business

· net - Network organizations

· ca - Canada

· th - Thailand

Because the Internet is based on IP addresses, not domain names, every Web Server

requires a Domain Name System server to translate domain names into IP addresses.

The Internet Protocol addresss is a 32- bit integer. If somebody wants to send a

message it is necessary to include the destination address, but people prefer to assign

machines pronounceable, easily remembered names (host names). For this reason the

Domain Name System is used. These logical names also allow independence from

knowing the physical location of a host. A host may be moved to a different network,

while the users continue to use the same logical name. The Domain Name System (DNS)

is a distributed database used by TCP/IP applications to map between hostnames and IP

addresses, and to provide electronic mail routing information. Each site (university

department, campus,company, or department within a company, for example) maintains

its own database of information and runs a server program that other systems across the

Internet can query. T...

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value in the response which specifies how long it guarantees the binding to remain.

DNS Formatting:

When the user wants to send a message, it invokes an application program and

supplies the name of a machine with which the application must communicate. The

application program must find the machine's IP address. It passes the domain name to a

local resolver (L.R.) and requests an IP address. The local resolver checks its cache and:

If the L.R. has an answer, it returns the answer.

If the L.R. hasn't one, it sends the message to the server. The server then returns a similar

message that contains the answer to the questions for which the server has bindings. If the

server can't answer, it sends responsive information about other servers that the client can

contact.

describes one name.

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