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The three parts of a domain name are the host record (www, @(none) or *(other)), the SLD (second level domain) and the TLD (top level domain) or ccTLD (country code top level domain). For example wiki.answers.com:wiki = host nameanswers = SLD (second level domain).com = TLD (top level domain)
the root domainDNS namespace: DNS is the name service provided by the Internet for TCP/IP networks. DNS is broken up into domains, a logical organization of computers that exist in a larger network. The domains exist at different levels and connect in a hierarchy that resembles the root structure of a tree. Each domain extends from the node above it, beginning at the top with the root-level domain. Under the root-level domain are the top-level domains, under those are the second-level domains, and on down into subdomains. DNS namespace identifies the structure of the domains that combine to form a complete domain name. For example, in the domain name sub.secondary.com, "com" is the top-level domain, "secondary" identifies the secondary domain name (commonly a site hosted by an organization and/or business), and "sub" identifies a subdomain within the larger network. This entire DNS domain structure is called the DNS namespace. The name assigned to a domain or computer relates to its position in the namespace.
A DNS root zone is the top-level DNS zone in a Domain Name System (DNS) hierarchy
A child domain is a member of a domain tree, but is not the root of the tree. The domain tree is the root of the tree.
Forest root domain
A designated port is the ports that goes from the root bridge switch to the edge of the network, so there is always a path from the root bridge to the other end. In contrast to root ports, which goes to the root bridge, instead of away from it. All ports that isn't root ports or designated ports are blocked.
I have a domain DomainA.com and X.DomainA.com is it consider a single domain or multiple domain?
The first DC in the domain.
A fully qualified domain name (FQDN), sometimes also referred as an absolute domain name, is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain and the root domain. A fully qualified domain name is distinguished by its unambiguity; it can only be interpreted one way.
The highest level of the DNS (Domain Name System) namespace is the root zone, which is represented by a dot (.) at the end of a domain name. It serves as the starting point for the hierarchical structure of the DNS, containing information about top-level domains (TLDs) such as .com, .org, and country code TLDs like .uk. The root zone is maintained by the Internet Corporation for Assigned Names and Numbers (ICANN) and is crucial for resolving domain names into IP addresses.
Them root server in general is . .com,.org etc but in your domain the first dc in the forest/domain is the one
forest root domain