Namespace
A namespace is a context within which the names of all objects must be unambiguously resolvable. For example, the internet is a single DNS name space, within which all network devices with a DNS name can be resolved to a particular address (for example, www.microsoft.com
resolves to 207.46.131.13).
A namespace can be flat or hierarchical. A flat namespace doesn't scale well because it can grow only so large before all available names are used up. Once a name is used more than once in a namespace, the namespace violates the unambiguously resolvable requirement.
A hierarchical namespace is divided into different areas, which can be thought of as sub-namespaces. Each area is its own sub-namespace within the overall namespace. Therefore, each object must have a unique name only within its sub-namespace in order to have an unambiguously resolvable name within the namespace hierarchy. Hierarchical namespaces, then, can scale to extremely large networks—as you add more objects to the overall name space, you have to find unique names for them within only the sub-namespace to which they belong.
All DNS namespaces are hierarchical. The sub-namespaces in the DNS hierarchical namespace are called domains. The unique name of a computer within a domain is called a relative distinguished name. Computers with the same relative distinguished name can exist in different sub-namespaces (domains) of the namespace hierarchy because they can be fully resolved to a unique object within the entire DNS hierarchy, using a fully-qualified domain name (FQDN). For example, you could have a server named server1 in the widgets.microsoft.com domain (the widgets.microsoft.com namespace), and you could have server1 in the gadgets.widgets.microsoft.com namespace. Because they are in different sub-namespaces in the hierarchical namespace, they can be resolved to different FQDNs—server1.widgets.microsoft.com and server1.gadgets.widgets.microsoft.com.