The Domain Name System (DNS) maps a host (computer) name to its IP address. When you specify a host by name, Cisco TCP/IP Suite uses DNS or host tables to map the host name to its IP address. The host name can be local to your organization or anywhere in the world, if your site is connected to the Internet. TCP/IP applications use DNS to convert host names to IP addresses, and vice versa. This conversion is called resolving.
Cisco TCP/IP Suite includes a DNS resolver which sends requests to another computer, called a DNS server, to resolve names into IP addresses. The DNS resolver can also send requests to the DNS server to resolve IP addresses to names.
You can configure Cisco TCP/IP Suite to use one or more DNS servers to resolve requests.
DNS does not control IP routing.
DNS centralizes information about host names and their IP addresses. Computers called DNS servers store the information so that your computer or any TCP/IP applications you use can access the information. If your computer needs information that is not on one DNS server, the server automatically requests the information from other DNS servers.
In DNS terminology, a domain is a group of computers. The domain administrator determines which computers are in the domain. A domain name identifies a domain and consists of words separated by dots. An example is YOYODYNE.COM.
Domain names are created by your domain administrator or may be special words used on the Internet. Domain names can pertain to a site, an organization, or to types of organizations.
When read right to left, the first word in the domain name is the top-level domain which identifies the function of an organization or specifies a country name code. In the name YOYODYNE.COM, .COM indicates an organization engaged in commerce. The top-level domain can also indicate a country, such as .CH for Switzerland. The name of the organization is to the left of the top-level domain, such as YOYODYNE. Any words to the left of the top-level domain are called subdomains. The left-most word in the domain name is the host name. For example, in OAK.YOYODYNE.COM, OAK is a host in the YOYODYNE.COM organization.
Domains and subdomains are organized in a hierarchical tree structure. Subdomains are set up and maintained by the domain administrator of each organization.
Just as the root directory in MS-DOS is expressed as a backslash, the root directory in DNS is expressed as a dot. Domains are analogous to directories and subdomains to directories within directories.
Top-level domains such as .ORG, .COM, and .EDU are typically used in the United States. Other countries group their domain names below their two-letter country code. Domains grouped under country codes include domains such as .CO for commercial and .AC for academic. In the United States, .US is occasionally used instead of another top-level domain name. Subdomains may provide additional geographic locations, such as .PALO-ALTO.CA.US.
A DNS server is any computer running DNS software that lets it communicate with other DNS servers and store address information for later retrieval. DNS servers are also called name servers. Your network administrator provides the IP address of the name server on your network.
Name servers come in five varieties:
A root name server is the start or base of the domain name tree. A root name server usually handles domains just below the root, such as top-level domains. Examples of top-level domains are .COM, .EDU, .US, and .CH. A root name server delegates authority to other primary name servers for subdomains, such as YOYODYNE.COM or CERN.CH.
A root name server is usually run by the InterNIC, the group responsible for naming domains on the Internet.
A primary name server has authority over one or more subdomains. A primary name server reads information about the domain over which it has authority from the zone file, a special file that describes DNS information about the domain and the hosts in that domain.
A primary name server is authoritative for domains and also caches DNS information.
A secondary name server for a domain receives information updates from the primary name server for that domain at regular intervals and stores this information on disk. A secondary server is also authoritative for the domain, and caches DNS information.
A caching-only name server caches (stores) domain information in memory for faster retrieval later. A caching-only name server is not authoritative for any domain. If a caching-only name server cannot resolve a request, it forwards the request to an authoritative name server for that domain.
A resolver sends requests for resolution to another server. Any name server that can handle the request returns the answer. Cisco TCP/IP Suite provides a DNS resolver. Resolvers do not cache DNS information.
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