DNS, Domains, and Zones
Domains starting at a top level (i.e., near the root) are particularly
important for DNS operation. These domains are generally called Top Level Domains (TLDs),
and TLDs are often referred to by specific names.
The entire name space is called the root
domain, and some common first-level domains are called generic top level domains or gTLDs.
Examples of gTLDs are the com, net, and org domains. At the first level, there are also percountry
domain names, which are called country code top level domains (or ccTLDs).
There is
no technical difference between gTLDs and ccTLDs in terms of the DNS protocol; the difference
is in who manages the corresponding name space.
An arbitrary connected subset of the name space can be a notion called a zone. Every node
in the name space belongs to one and only one of the zones, which allows a zone to define
an administration boundary of a particular part of the name space.
Note that a zone is not
necessarily a sub-tree, and does not necessarily equal the domain of the same name. In fact,
the corresponding zones for most top-level domains are not equal to the domains; in particular,
the root zone effectively consists of the root node only.
Each zone is served by one or more nameservers. A nameserver.
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Domain Name Lookup - Domain Name Search
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Domain Name System (DNS)
The top-notch step in resolving host names to TCP/IP addresses
is using DNS.
The Domain Name System (DNS) is a way to resolve meaningful
and easy-to-remember names to IP addresses.
Because millions of
sites are connected to the Internet, maintaining one central list of the nameto-
IP-address relationships across the Internet is unrealistic. The DNS system
was designed to coordinate and distribute the resolution load.
The two major tasks that DNS provides are:
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IP address resolution to hosts on the Internet, for local hosts
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IP address resolution to hosts on the local network, for other hosts on
the Internet
NameServer
Each zone is served by one or more nameservers. A nameserver of a zone maintains domain
names within the zone, and responds to database queries for the names.
Nameservers are
often called authoritative (DNS) servers. Nameservers of the root zone and gTLD zones are
called the root servers and the gTLD servers, respectively. One single nameserver can serve
for multiple different zones. For example, com and net zones are served by the same set of
nameservers.
Many zones have more than one authoritative server, particularly in the case of top-level
zones, to provide redundancy and improve stability.
Usually only one of them maintains the
master database, which is called the primary (or master) server of the zone. Other nameservers,
called secondary (or slave) servers, periodically synchronize with the primary server to provide
coherent behavior. This process is called a zone transfer.
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