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The TCP/IP Protocol Suite

Domain Name System (DNS)

The previously discussed protocols do not use any concept of a host name. These protocols always use 32-bit IP addresses to locate source and destination hosts. Of course, no one wants to specify a remote computer using an address such as 192.22.31.05. The Domain Name System (DNS) maps IP addresses to alphabetic names. One of the most important features of DNS is distributed management.

Each organization has the ability to control names within its own domain. Domains are arranged in a hierarchy. For example, the XYZ Company, Inc., may have names all ending in the following:

.xyz.com

com the final section of the name, is a higher-level domain used to group commercial organizations.
xyz the second section of the name, is the designated name of the organization.

The names could be further divided into several groups such as the following:
unx
vm
dev

For example,

abcvm.vm.xyz.com

might be the primary VM system at the XYZ Company, Inc. DNS enables you to use a File Transfer Program command such as

 ftp abcvm.vm.xyz.com

instead of

ftp 123.45.67.89

when transferring a file to this VM system.

Although it is possible to locate the mapping of host addresses to host names in a file (for example, /etc/hosts on UNIX), DNS is more versatile than a system that maps addresses to names in a file. Under a system that maps names to addresses, the file containing the mapped names and addresses: must be replicated on every host, does not have the capacity to contain the mappings for all computers on a system as large as the Internet, and cannot be updated on a real-time basis.

DNS uses server processes called name servers to stay current with the names assigned within a particular domain. The network administrator provides the name servers with configuration files. Each configuration file contains the mapping for the domain that it controls. Name servers in a particular domain can refer to the addresses of name servers for higher- and lower-level domains if the configuration files that they control do not contain a particular name or address.

Name servers typically run on only a few machines in an organization. Programs can use a set of routines, known as the resolver, to query their organization's name server. The resolver routines are associated with the application and provide all the message formatting and TCP or UDP communications logic necessary to talk to their organization's name server.

DNS is general enough to allow distributed management of other types of information, such as mailbox locations, and it does not require any correspondence between domains and IP addresses or physical network connections.


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