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However, the limitations of a flat naming scheme became clear before the end of the ARPANet and :rfc:`819` proposed a hierarchical naming scheme. While :rfc:`819` discussed the possibility of organizing the names as a directed graph, the Internet opted for a tree structure capable of containing all names. In this tree, the top-level domains are those that are directly attached to the root. The first top-level domain was `.arpa` [#fdnstimeline]_. This top-level name was initially added as a suffix to the names of the hosts attached to the ARPANet and listed in the `hosts.txt` file. In 1984, the `.gov`, `.edu`, `.com`, `.mil` and `.org` generic top-level domain names were added. :rfc:`1032` proposed the utilization of the two letter :term:`ISO-3166` country codes as top-level domain names. Since :term:`ISO-3166` defines a two letter code for each country recognized by the United Nations, this allowed all countries to automatically have a top-level domain. These domains include `.be` for Belgium, `.fr` for France, `.us` for the USA, `.ie` for Ireland or `.tv` for Tuvalu, a group of small islands in the Pacific or `.tm` for Turkmenistan. The set of top-level domain-names is managed by the Internet Corporation for Assigned Names and Numbers (:term:`ICANN`). :term:`ICANN` adds generic top-level domains that are not related to a country and the `.cat` top-level domain has been registered for the Catalan language. There are ongoing discussions within :term:`ICANN` to increase the number of top-level domains.
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Part 3: Practice
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Each top-level domain is managed by an organization that decides how sub-domain names can be registered. Most top-level domain names use a first-come first served system, and allow anyone to register domain names, but there are some exceptions. For example, `.gov` is reserved for the US government, `.int` is reserved for international organizations and names in the `.ca` are mainly `reserved <http://en.wikipedia.org/wiki/.ca>`_ for companies or users that are present in Canada.
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Appendices
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The syntax of the domain names has been defined more precisely in :rfc:`1035`. This document recommends the following :term:`BNF` for fully qualified domain names (the domain names themselves have a much richer syntax).
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Indices and tables
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BNF of the fully qualified host names
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:ref:`genindex`
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This grammar specifies that a host name is an ordered list of labels separated by the dot (`.`) character. Each label can contain letters, numbers and the hyphen character (`-`) [#fidn]_. Fully qualified domain names are read from left to right. The first label is a hostname or a domain name followed by the hierarchy of domains and ending with the root implicitly at the right. The top-level domain name must be one of the registered TLDs [#ftld]_. For example, in the above figure, `www.computer-networking.info` corresponds to a host named `www` inside the `computer-networking` domain that belongs to the `info` top-level domain.
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:ref:`search`
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Some visually similar characters have different character codes
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The Domain Name System was created at a time when the Internet was mainly used in North America. The initial design assumed that all domain names would be composed of letters and digits :rfc:`1035`. As Internet usage grew in other parts of the world, it became important to support non-ASCII characters. For this, extensions have been proposed to the Domain Name System :rfc:`3490`. In a nutshell, the solution that is used to support Internationalized Domain Names works as follows. First, it is possible to use most of the Unicode characters to encode domain names and hostnames, with a few exceptions (for example, the dot character cannot be part of a name since it is used as a separator). Once a domain name has been encoded as a series of Unicode characters, it is then converted into a string that contains the ``xn--`` prefix and a sequence of ASCII characters. More details on these algorithms can be found in :rfc:`3490` and :rfc:`3492`.
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