There are many DNS servers throughout the Internet. However, each DNS server
stores only a portion of the entire Internet namespace. A DNS hierarchy enables
DNS servers to find their neighbors and ask each other for information about a
specific host.
A domain is a label in the DNS hierarchy.
Each node in the
DNS hierarchy represents a domain. Domains under the top-level domains
represent individual organizations or entities. These domains can be further
divided into subdomains to ease administration of an organization's host
computers.
Top-level domain names are part of most URLs. For example,
".com," ".edu," ".net," ".gov," and
".org" are top-level domain names. These top-level domains contain
the basis for the rest of the domain naming structure. Individual organizations
are granted second-level domain names within one or more of these top-level
domains. Because names have to be unique in a domain, they must be registered.
When an organization wishes to acquire a second-level domain name, it must
submit a request to one of the Registrars accredited by the Internet
Corporation for Assigned Names and Numbers (ICANN). If the domain name is
available and the Registrar does not have a problem with the name, it is
assigned to the organization in exchange for a fee. The organization itself is
responsible for assigning third-level and lower domains.
How DNS is
Resolved
In Figure
, the
client makes a request to the corporate DNS server. The DNS server checks its
cache to see if the query has already been resolved. In this situation, the
corporate DNS server has no record of this query. Therefore, the corporate DNS
switches roles and now acts as a client and issues an iterative query to the
local ISP.
The ISP name server has no record of this resolved request. The ISP server
replies back with a hint to query the root domain server.
The DNS server
issues an iterative query at the top of the DNS hierarchy to the root level
server. After each query and response the server goes down the DNS tree until
it finally finds the correct resolved name.
Nslookup
The most
effective command for testing and resolving DNS issues is the
nslookup command.

If the
lookup request fails, nslookup prints an error message.
Figure
lists possible
error messages.
DNS and Routers
A router can be configured to
use DNS lookups so that ping or
traceroute commands can be used with a hostname rather than
an IP address. Use the commands in Figure
to do so.