| Parts Of Domain Name |
A domain name usually consists of two or more parts (technically labels),
separated by dots. For example wikipedia.org.
- The rightmost label conveys the top-level domain (for example,
the address en.wikipedia.org has the top-level domain org).
- Each label to the left specifies a subdivision or subdomain of
the domain above it. Note that "subdomain" expresses relative dependence,
not absolute dependence: for example, wikipedia.org comprises a
subdomain of the org domain, and en.wikipedia.org
comprises a subdomain of the domain wikipedia.org. In theory, this
subdivision can go down to 127 levels deep, and each label can contain up to
63 characters, as long as the whole domain name does not exceed a total
length of 255 characters. But in practice some domain registries have
shorter limits than that.
- A hostname refers to a domain name that has one or more
associated IP addresses. For example, the en.wikipedia.org and
wikipedia.org domains are both hostnames, but the org domain
is not.
The Domain Name System consists of a hierarchical set of DNS servers.
Each domain or subdomain has one or more authoritative DNS servers that
publish information about that domain and the name servers of any domains
"beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy
of domains. At the top of the hierarchy stand the root nameservers: the
servers to query when looking up (resolving) a top-level domain name (TLD).
Iterative and recursive queries:
- An Iterative query is one where the DNS server may provide a partial
answer to the query (or give an error). DNS servers must support
non-recursive queries.
- A recursive query is one where the DNS server will fully answer the
query (or give an error). DNS servers are not required to support
recursive queries and both the resolver (or another DNS acting recursively
on behalf of another resolver) negotiate use of recursive service using bits
in the query headers.
Address resolution mechanism
- (This description deliberately uses the fictional .example TLD
in accordance with the DNS guidelines themselves.)
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).
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