Historically, TLDs were implemented in order to divide the namespace into autonomous areas for different sectors of the economy; for instance, government, education, commercial and nonprofit organizations, and the military all needed their own subdivisions of the namespace, independent of other participants in the namespace. Top-level domains are governed by the Internet Corporation for Assigned Names and Numbers (ICANN), and the number of TLDs available for public use is limited. Queries between TLDs are serviced by the root domain servers, denoted as the dot before the TLD. The most commonly used North American TLDs are as follows:
■ com Commercial organizations
■ edu Educational institutions
■ org Nonprofits
■ net Network support centers and network service providers
■ two-letter ISO country code Such as "us" for United States and "ca" for Canada. Each country has its own country-code TLD.
Thus, if you see a hostname or a domain name ending with .edu, it is most likely an educational institution in the United States. Likewise, a name ending with .ca more than likely represents an entity or individual who is incorporated or domiciled in Canada.
The second-level domain (SLD) is the next portion of an FQDN after the TLD, going from the root dot to the left. Continuing with our example, flexecom would be the SLD in www.flexecom.com. It wasn't long before short, descriptive, and intuitive SLD names were depleted, and remembering hostnames with three or four domain levels is tantamount to remembering IP addresses. So ICANN authorized a new set of TLDs in 2001, namely, .info, .biz, .name, .news, .pro, .museum, .aero, .inc, .mail, .games, .learn, and .coop. You can find a complete listing of new TLDs at http:// www.icann.org/tlds.
To distinguish resources in local area networks from those located in the public portion of the namespace (i.e., on the Internet), companies are allowed to use the .local TLD name, which is invalid in the public sector, but it doesn't need to be registered with ICANN. For example, say we want to set up another namespace to be used locally in our imaginary organization. We could use flexecom.local, while flexecom.com is still maintained in parallel to support external requests that can be resolved using public namespace mechanisms outside of the private network. The .local TLD does not belong to the same root, so it cannot be resolved on the Internet. Technically, companies may use any non-registered TLD for internal naming only.
DNS is also used for reverse resolution—that is, resolution of IP addresses into fully qualified domain names. To facilitate this, DNS uses a special service zone, in-addr.arpa. Although this may seem like a second-level domain, the in-addr.arpa zone is as important from the reverse name resolution perspective as a top-level domain is from the name resolution perspective. A reverse name resolution zone is named using octets denoting classified network ID numbers, ordered in reverse if you read from left to right. For example, IP address 192.168.10.1 is a class C IP address, where 192.168.10 represents the network ID and .1 represents the host ID.
So if we wanted to set up a zone for reverse mapping of IP addresses to hostnames, we would name the domain 10.168.192.in-addr.arpa. In essence, if you request the hostname of the 192.168.10.1 machine, you request addressing information for node "1" from domain 10.168.192.in-addr.arpa. Similarly, in the case of a class A address, such as 10.1.1.1, we would need the 10.in-addr.arpa domain, and in a class B address, such as 172.16.30.11, it would be 16.172.in-addr.arpa. In the latter example, the 16.172.in-addr.arpa domain would need to contain a 30. subdomain, where information about host "11" would be stored.
The terms "domain name" and "SLD" can be used interchangeably, although the vast majority of technicians identify with the former. Domain name such as "flexecom.com" must be registered with any of the domain registrars to prevent name duplication and to set them up on the TLD servers, which in turn enables domain name resolution by third parties on the Internet. Uniqueness of third-level domains and lower is not crucial for name resolution on the Internet as a whole and is usually the responsibility of the company or organization that owns the respective domain name.
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