Domain Services
On the Internet, the Domain Name System (DNS) associates various sorts of information with so-called domain names; most importantly, it serves as the "phone book" for the Internet: it translates human-readable computer hostnames, e.g. en.wikipedia.org, into the IP addresses that networking equipment needs for delivering information.
The most basic use of DNS is to translate hostnames to IP addresses. It is in very simple terms like a phone book. For example, if you want to know the internet address of en.wikipedia.org, DNS can be used to tell you it's 66.230.200.100. DNS also has other important uses.
The practice of using a name as a more human-legible abstraction of a machine's numerical address on the network predates even TCP/IP, and goes all the way to the ARPAnet era.
The domain name space consists of a tree of domain names. Each node or leaf in the tree has one or more resource records, which hold information associated with the domain name. The tree sub-divides into zones. A zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. (Note that a single nameserver can host several zones.)
A domain name usually consists of two or more parts (technically labels), separated by dots. For example wikipedia.org.
In theory a full host name may have several name segments, (e.g ahost.ofasubnet.ofabiggernet.inadomain.example). In practice, in the experience of the majority of public users of Internet services, full host names will frequently consist of just three segments (ahost.inadomain.example, and most often www.inadomain.example).
Name servers in delegations appear listed by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred.
After having found a list of addresses of servers capable of answering queries about (eg.) the .example TLD, the local resolver will not make a query for .example again until the validity of the currently known list expires. The same applies for all subdomains. Once a response goes into cache, the resolver will consult its cached (stored) answer; only when the TTL expires (or when an administrator manually flushes the response from the resolver's memory) will the resolver contact the DNS server for the same information.
As a noteworthy consequence of this distributed and caching architecture, changes to the DNS do not always take effect immediately and globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP address at 12:00pm will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to the DNS: not everyone is necessarily seeing the same thing you're seeing. RFC 1537 helps to convey basic rules for how to set the TTL.
Users generally do not communicate directly with a DNS resolver. Instead DNS resolution takes place transparently in client applications such as web browsers, mail clients, and other Internet applications. When a request is made which necessitates a DNS lookup, such programs send a resolution request to the local DNS resolver in the operating system which in turn handles the communications required
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