The Domain Name System uses a tree (or hierarchical) name structure. At the top of the tree is the root followed by the Top Level Domains (TLDs) then the domain-name and any number of lower levels each separated with a dot.

NOTE: The root of the tree is represented most of the time as a silent dot ('.') but there are times as we shall see later when it VERY important.

Top Level Domains (TLDs) are split into two types:

1. Generic Top Level Domains (gTLD) .com, .edu, .net, .org, .mil etc.

2. Country Code Top Level Domain (ccTLD) e.g. .us, .ca, .tv , .uk etc.

Country Code TLDs (ccTLDs) use a standard two letter sequence defined by ISO 3166. Since 2004 gTLDs now have a sub-category known as sTLDs (Sponsored TLDs) which implies they have limited registration, examples of sTLDs include .aero, .museum, .travel, and .jobs whereas the normal gTLDs typically have open registration requirements.

Figure 1-1 shows this diagrammatically.

Figure 1-1 Domain Structure and Delegation

What is commonly called a 'Domain Name' is actually a combination of a domain-name and a TLD and is written from LEFT to RIGHT with the lowest level in the hierarchy on the left and the highest level on the right.

domain-name.tld # example.com

In the case of the gTLDs, such as .com, .net etc., the user part of the delegated name – the name the user registered – is a Second Level Domain (SLD). It is the second level in the hierarchy. The user part is therefore frequently simply referred to as the SLD. So the the Domain Name in the example above can be re-defined to consist of:

sld.tld # example.com

The term Second Level Domain (SLD) is much less useful with ccTLDs where the user registered part is typically the Third Level Domain, for example:

example.co.uk
example.com.br

The term Second Level Domain (SLD) provides technical precision but can be confusing – unless the precision is required we will continue to use the generic term Domain Name or simply Domain to describe the whole name, for instance, what this guide calls a Domain Name would be example.com or example.co.uk.

The concepts of Delegation and Authority lie at the core of the domain name system hierarchy. The Authority for the root domain lies with Internet Corporation for Assigned Numbers and Names (ICANN). Since 1998 ICANN, a non-profit organisation, has assumed this responsibility from the US government.

The gTLDs are authoritatively administered by ICANN and delegated to a series of accredited registrars. The ccTLDs are delegated to the individual countries for administration purposes. Figure 1.0 above shows how any authority may in turn delegate to lower levels in the hierarchy, in other words it may delegate anything for which it is authoritative. Each layer in the hierarchy may delegate the authoritative control to the next lower level.

In the case of ccTLDs countries like Canada (ccTLD .ca) and the US (ccTLD .us) and others with federal governments have decided that they will administer at the national level and delegate to each province (Canada) or state (US) a two character province/state code, for example, .qc = Quebec, .ny = New York, md = Maryland etc.. Thus mycompany.md.us is the Domain Name of mycompany which was delegated from the state of MaryLand in the US.

Countries with more centralized governments, like the UK and others, have opted for functional segmentation in their delegation models, for example, .co = company, .ac = academic etc.. Thus mycompany.co.uk is the Domain Name of mycompany registered as a company from the UK registration authority.

Delegation within any domain may be almost limitless and is decided by the delegated authority, for example, the US and Canada both delegate city within province/state domains thus the address (or URL) tennisshoes.nb.us is the town of Tennis Shoes in the State of Nebraska in the United States and we could even have mycompany.tennisshoes.nb.us.

By reading a domain name from RIGHT to LEFT you can track its delegation. This unit of delegation can also be referred to as a zone in standards documentation.

From our reading above we can see that www.example.com is built up from www and example.com. The Domain-Name example.com part was delegated from a gTLD registrar which in turn was delegated from ICANN.

The www part was chosen by the owner of the domain since they are now the delegated authority for the example.com name. They own EVERYTHING to the LEFT of the delegated Domain Name.

The leftmost part, www in this case, is called a host name. By convention (but only convention) web sites have the 'host' name of www (for world wide web) but you can have a web site whose name is fred.example.com – no-one may think of typing this into their browser but that does not stop you doing it! Equally you may have a web site whose access address (URL) is www.example.com running on a server whose real name is mary.example.com. Again this is perfectly permissable. In short the host part may refer to a real host name or a service name such as www. Since the domain owner controls this process it's all allowed.

Every computer, or service, that is addressable (has a URL) via the Internet or an internal network has a host name part, here are some more illustrative examples:

www.example.com – the company web service
ftp.example.com – the company file transfer protocol server
pc17.example.com – a normal PC
accounting.example.com – the main accounting system

A host name part must be unique within the Domain Name but can be anything the owner of example.com wants.

Finally lets look at this name:

www.us.example.com

From our previous reading we figure its Domain Name is example.com, www probably indicates a web site, which leaves the us part.

The us part was allocated by the owner of example.com (they are authoritative) and is called a sub-domain. In this case the delegated authority for example.com has decided that their company organization is best served by a country based sub-domain structure. They could have delegated the responsibility internally to the US subsidiary for administration of this sub-domain, which may in turn have created a plant based structure, such as, www.cleveland.us.example.com could indicate the web site of the Cleveland plant in the US organisation of example.com.

To summarise the OWNER can delegate, IN ANY WAY THEY WANT, ANYTHING to the LEFT of the Domain Name they own (were delegated). The owner is also RESPONSIBLE for administering this delegation which means running, or delegating the task of running, a DNS containing Authoritative information (or records) for their Domain Name (or zone).

Note: Names such as www.example.com and www.us.example.com are commonly – but erroneously – referred to as Fully Qualified Domain Names (FQDN). Technically an FQDN unambiguously defines a name from any starting point to the root and as such must contain the normally silent dot at the end. To illustrate "www.example.com." is an FQDN "www.example.com" is not.