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Subject : DOMAIN NAME SYSTEM
DOMAIN NAME SYSTEM
In order to associate the contents of the site with its address, a domain naming system was developed. A domain is a group of computers that are associated by their geographical location or their business type. A domain name is a string of characters and/or numbers, usually a name or abbreviation that represents the numeric address of an Internet site. There are more than 200 top-level domains on the Internet, examples of which include the following:
.us - United States
.uk - United Kingdom
There are also generic names, examples of which include the following:
.edu - educational sites
.com - commercial sites
.gov - government sites
.org - non-profit sites
.net - network service
The domain name server (DNS) is a device on a network. It responds to requests from clients to translate a domain name into the associated IP address. The DNS system is set up in a hierarchy that creates different levels of DNS servers.
If a local DNS is able to translate a domain name into its associated IP address, it does so, and returns the result to the client. If it cannot translate the address, it passes the request up to the next higher-level DNS on the system, which then tries to translate the address. If the DNS at this level is able to translate the domain name into an associated IP address, it does so, and returns the result to the client. If not, it sends the request to the next higher level. This process repeats itself until the domain name has been translated, or the top-level DNS has been reached. If the domain name cannot be found on the top level DNS, it is considered to be an error and the corresponding error message is returned.
Any type of application that uses domain names to represent IP addresses, uses the DNS to translate that name into its corresponding IP address.
You select network applications based on the type of work you need to accomplish. A complete set of application layer programs is available to interface with the Internet. Each application program type is associated with its own application protocol.
• The World Wide Web uses the HTTP protocol.
• Remote access programs use the Telnet protocol for directly connecting to remote resources.
• E-mail programs support the POP3 application layer protocol for electronic mail.
• File utility programs use the FTP protocol for copying and moving files between remote sites.
• Network data gathering and monitoring use the SNMP protocol.
It is important to re-emphasize the fact that the application layer is just another protocol layer in the OSI or TCP/IP models. The programs interface with application layer protocols.
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