DNS - Domain Name System. Basics, Namespace structure

Domain Namespace

Domain Namespace is, in fact, hierarchical and logical tree of domain names. Except "global" namespace, organizations can create their own private namespaces that are not visible from Internet. Domain namespace consists of root domain (which is nameless) from which we have generic top-level Internet DNS domains (gTLDs) and country code Internet DNS domains (ccTLDs), then in lower branches or nodes we have our domains and sub-domains.



Structure of Domain Namespace

Each node in the DNS tree represents a DNS name. Every node in the DNS domain tree can be identified by a fully qualified domain name (FQDN). The FQDN is absolute domain path, like: www.google.com. where www is name of some machine, google is second level domain and com is top-level domain. You will often see trailing dot after top-level domain, this dot represents root.



FQDNs have naming restriction that allow characters a-z, A-Z, 0-9 and minus sign, but dot (.) isn't allowed because it separates domain name labels.

Domain Name Syntax

If you pick several domains and look at them simultaneusly, you don't have to be genius to see some rules in naming. But sometimes is very difficult to see all rules, so here is the list of them:

The right-most label conveys the top-level domain, for example, the domain name www.example.com belongs to the top-level domain com.
• The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example: the label example specifies a subdomain of the com domain, and www is a sub domain of example.com. This tree of subdivisions may have up to 127 levels.
• Each label may contain up to 63 characters. The full domain name may not exceed a total length of 253 characters in its external dotted-label specification. In the internal binary representation of the DNS the maximum length requires 255 octets of storage. In practice, some domain registries may have shorter limits.
• DNS names may technically consist of any character representable in an octet. However, the allowed formulation of domain names in the DNS root zone, and most other sub domains, uses a preferred format and character set. The characters allowed in a label are a subset of the ASCII character set, and includes the characters a - z, A - Z, digits 0 - 9, and the hyphen. This rule is known as the LDH rule (letters, digits, hyphen). Domain names are interpreted in case-independent manner. Labels may not start or end with a hyphen.
• A hostname is a domain name that has at least one IP address associated. For example, the domain names www.example.com and example.com are also hostnames, whereas the com domain is not.
  
  

Zones

When we talk or read about DNS, we often have one word repeating - zone. Administratively, each level or node in the hierarchy represents a potential boundary of authority for management of the name space. The authority over every level in every branch of the name space tree is delegated to a legal entity or organization, such as a top-level country's domain registry, or a company or individual registered to use a given sub-domain in the system. 

These administrative spaces or portions of the domain name system are termed "DNS zones". DNS zones may consist of only one domain, or may comprise many domains and sub-domains, depending on the administrative authority delegated to the manager. Each manager may further delegate authority over a sub-space of its delegation to other parties.
Windows Server 2003 introduced new type of zone - stub zone. Stub zone provides redundancy in way that only contains resource records of authoritative servers. In other words, stub zones serve as a copy of the referred zone that just contains the records essential to identify the authoritative DNS servers for the referred zone. In this manner we save bandwidth.

In MS Windows Server 2008, Microsoft also introduced GNZ, also known GlobalNames Zone. It supports static, global records with single-label names, not unlike WINS, but in a purely DNS environment. In contrast with WINS the GlobalNames zone is not for workstation name resolution, and does not support dynamic updates; the supported model is that system administrators put the addresses of enterprise servers or web sites to the GNZ centrally.

In next part of tutorial we will find out which type of DNS servers we have...