The DNS namespace is a hierarchical space. A hierarchy is defined as a structure in which an entity is subordinate to the entity above it. Many of us are familiar with hierarchies such as the military, where a private is subordinate to a corporal, who is subordinate to a sergeant, and so forth. In the DNS hierarchy, the DNS name is divided into separate levels, each denoted by a dot or period (.).The top of the hierarchy is the right-most part of the name and the lowest level of the hierarchy is the left-most part of the name.
Every node in the hierarchy has a name, referred to as a label, which can be zero to 63 octets in length. This is more commonly referred to as "up to 63 characters" because alphanumeric characters each are denoted within an octet. The 63 character limit is also referred to as 63 bytes, since a byte is 8 bits, or an octet. Nodes on the same branch cannot have the same name, but nodes on separate branches can.
The domain name for a specific node is the list of labels along the path from the root to the specific node.The full list of labels for a particular node is referred to as a FQDN. It is considered fully qualified because the entire list of labels is shown, leaving no doubt as to the entire path through the hierarchy to the specific node. Since we read domain names from left to right, we start with the most specific name and move up the hierarchy toward the more generic top level domain (TLD).This tree-like structure is shown in Figure 5.1, which shows only a few of the total number of TLDs now available.
Figure 5.1 DNS Name Structure
Figure 5.1 DNS Name Structure
The root domain is denoted with the use of a dot (.) and has a length of zero characters. Technically, when written, all domain names end with the root character, though it is rarely denoted this way in common practice. Thus, a well-known domain name like microsoft.com should more correctly be denoted as microsoft.com., with the final dot shown at the right end of the name. After the root, we move to the right, to the TLD.There are three types of TLDs: ARPA, Generic, and Country Codes.
The ARPA TLD is reserved for reverse name lookups and is discussed later in this chapter.
There were originally seven generic TLDs, as defined in RFC 1034: com, net, org, edu, mil, gov, and int. The seven TLDs were used by a variety of organizations, although the designations originally were intended for use by particular types of organizations.Table 5.1 shows the original intended use of the seven TLDs.
Table 5.1 Original Top Level Domain Designations
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