Below the top-level domains in the
domain name hierarchy are the second-level domain
(SLD) names. These are the names directly to the left of .com, .net, and the
other top-level domains. As an example, in the domain example.co.uk, co
is the second-level domain.
Next are third-level domains, which
are written immediately to the left of a second-level domain. There can be
fourth- and fifth-level domains, and so on, with virtually no limitation. An
example of an operational domain name with four levels of domain labels is sos.state.oh.us.
Each label is separated by a full stop (dot). 'sos' is
said to be a sub-domain of 'state.oh.us', and 'state' a sub-domain of 'oh.us',
etc. In general, subdomains are domains
subordinate to their parent domain.
An example of very deep levels of subdomain
ordering are the IPv6 reverse resolution DNS zones, e.g.,
1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa, which
is the reverse DNS resolution domain name for the IP address of a loopback interface, or the localhost name.
Second-level (or lower-level,
depending on the established parent hierarchy) domain names are often created
based on the name of a company (e.g., bbc.co.uk), product or service
(e.g. hotmail.com).
Below these levels, the next domain name component
has been used to designate a particular host server. Therefore, ftp.example.com
might be an FTP server, www.example.com would be a World Wide Web server, and mail.example.com
could be an email server, each intended to perform only the implied function.
Modern technology allows multiple physical servers with either different (cf. load balancing)
or even identical addresses (cf. anycast) to serve a single
hostname or domain name, or multiple domain names to be served by a single
computer. The latter is very popular in Web hosting service
centers, where service providers host the websites of many organizations on
just a few servers.
The hierarchical DNS labels or components of domain names are
separated in a fully qualified name by the full stop (dot, .).
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