DNS overview

While TCP/IP applications refer to host computers by their IP addresses, it is easier to use host names. To enable the use of host names in a network, the Domain Name System (DNS) translates host names to IP addresses. Mapping must be consistent across the network to ensure interoperability. DNS provides the host name-to-IP address mapping through network server hosts called domain name servers. For detailed information about name servers, see Domain name servers. DNS can also provide other information about server hosts and networks such as the TCP/IP services available at a server host and the location of domain name servers in a network.

DNS organizes the hosts in a network into domains. A domain is a group of hosts that share the same name space in the domain hierarchy and are usually controlled within the same organization. Domains are arranged in a hierarchy. A special domain known as the root domain exists at the top of the hierarchy. The root domain servers store information about server hosts in the root domain and the name servers in the delegated, top-level domains, such as com (commercial), edu (education), and mil (military). The name servers in the top-level domain, in turn, store the names of name servers for their delegated domains, and so on.

The complete name of a host, also known as the fully qualified domain name (FQDN), is a series of labels separated by dots or periods. Each label represents an increasingly higher domain level within a network. The complete name of a host connected to one of the larger networks generally has more than one subdomain, as shown in the following examples:

host1.subdomain2a.subdomain2.rootdomain
user4720.eng.mit.edu

A domain name server requires the FQDN. The client resolver combines the host name with the domain name to create the FQDN before sending the name resolution request to the domain name server.

DNS also provides IP address-to-host name mapping. The DNS defines a special domain called in-addr.arpa to translate IPv4 addresses to host names, and the ip6.int and ip6.arpa domains for IPv6 address-to-host name translation. This kind of mapping is useful for producing output (host names) that is easy to read. An in-addr.arpa name is composed of the reverse octet order of an IP address concatenated with the in-addr.arpa string. For example, a host named Host1 has 9.67.43.100 as an IP address. The in-addr.arpa domain translates the Host1 IP address 9.67.43.100 to 100.43.67.9.in-addr.arpa.

A system administrator can name the host systems and domains in the local, private network with any name you want, but to link with name servers in a public network like the Internet, you need to determine which domain you want to be in (which parent domain) and then contact the registrar in that domain to register the names and IP addresses of your name servers. This ensures that queries from outside the domain being defined can be answered by this name server if need be.

Note: Contact the InterNetwork Information Center (InterNIC) for more information about Internet registration. You can contact InterNIC by pointing your web browser at http://www.internic.net.