Understanding DNS Hierarchy: Complete Guide to Domain Name Space
Understand the DNS hierarchy structure, from root level domains down to individual hosts. Learn how domain names are organized, interpreted, and resolved through the inverted tree structure of the domain name space.
What is DNS Hierarchy?
The DNS hierarchy, also called the domain name space, is an inverted tree structure. The DNS hierarchy tree has a single domain at the top of the structure called the root domain – indicated by the "." as we have mentioned. Below the root domain are the top-level domains that divide the DNS hierarchy into segments containing second-level domains, sub-domains, and hosts.
Think of DNS hierarchy as an upside-down tree where the root is at the top and branches extend downward to reach specific hosts. This structure enables efficient name resolution and delegation of authority.
Five Levels of DNS Hierarchy
The DNS hierarchy is comprised of the following five levels:
| Level | Description | Example |
|---|---|---|
| 1. Root Level Domain | The top of the hierarchy, represented by a dot (.) | . |
| 2. Top Level Domains (TLD) | First level below root (organizational or geographic) | .com, .org, .uk, .ca |
| 3. Second Level Domains (SLD) | Registered domain name below TLD | rjsnetworkcloudacademy |
| 4. Subdomains | Optional third-level domains for organization | www, blog, mail |
| 5. Hosts | Individual devices or servers | myserver, webserver1 |
Within the hierarchy, the resolution process starts at the Root Level DNS, the Top-Level DNS, working its way down to the Second Level DNS, then through any number of sub-domains until we get to the actual hostname we want to resolve into an IP address.
DNS Architecture
Domain Name formats are examined from right to left because that is the direction that a domain name is interpreted. This is contrary to the way we type a domain name into a browser from left to right.
For instance, www.rjsnetworkcloudacademy.com domain names are interpreted by DNS the other way around, from right to left.
Components of a Domain Name (Labels)
- The first component for domain name starting from right to left is the Root represented by a dot.
- After the Root comes, the Top-Level Domain (TLD). We will not go into depth now at this stage, since there's going to be an entire module dedicated to TLD further.
- What follows a Top-level domain is the second level domain, which in this case is rjsnetworkcloudacademy.
- The second level domain, along with a Top-level domain, make the Zone apex otherwise known as Naked domain or Apex domain.
- Finally, the third level domain is usually something like www, which is more of a naming convention rather than a necessity, if it came down to creating a domain name for, say, your website.
- The components of a domain name are known as Labels
- Root is said to have a Label of Null
- Each label is a subdomain of its parent domain
- For instance,
wwwis a subdomain ofrjsnetworkcloudacademy.com, whilerjsnetworkcloudacademyis a subdomain of the.comtop-level domain
3. Domain Name Syntax Rules
A few more things to keep in mind in relation to the syntax of a domain name:
| Rule | Description |
|---|---|
| Label Length | Each label can be up to 63 characters long |
| Allowed Characters | A-Z uppercase/lowercase, digits 0-9, and hyphen (-). This is known as the LDH rule (Letters, Digits, Hyphen) |
| Hyphen Placement | Labels may not start or end with a hyphen |
| TLD Restriction | A top-level domain name should not be all numeric |
| Subdomain Limit | No maximum limit on the number of subdomains |
| Total Length | A domain name can be up to 255 characters in total, including the dots |
4. Root Level Domains
The DNS root level is the highest in the DNS hierarchy tree because it is the first step in resolving a domain name. The root DNS server is the DNS for the root zone. It handles requests for records in the root zone and answers other requests by providing lists of authoritative name servers for the appropriate TLD (top-level domain). These are the authoritative nameservers that serve the DNS root zone. These servers contain the global list of the top-level domains.
Root Zone Contents
The root zone contains the following:
- Organizational hierarchy – such as .com, .net, .org, .edu
- Geographic hierarchy – such as .ca, .uk, .fr, .pe
Currently, there are 13 root name servers specified, with logical names in the form "letter.root-servers.net", where letter ranges from "A" to "M" and represent companies like Verisign, University of Maryland, NASA, and The Internet Corporation for Assigned Names and Numbers (ICANN).
5. Top-Level Domains (TLD)
The next level in the DNS hierarchy is Top-Level Domains or TLDs, for short. There are over 1000 TLDs covering everything from ".abb" to ".zw" and still growing. As we have seen, the TLDs are classified into two subcategories: organizational hierarchy and geographic hierarchy.
| TLD Category | Description | Examples |
|---|---|---|
| Organizational Hierarchy | Divided into domains for commercial enterprises, government entities, educational institutions, and nonprofit organizations | .com (commercial) .gov (government) .edu (education) .org (organization) |
| Geographic Hierarchy | Represents the country where the domain is hosted (country-code TLDs) | .ca (Canada) .uk (United Kingdom) .au (Australia) .aq (Antarctica) |
| Combined Hierarchy | Organizations using both organizational and geographical hierarchies for local customers | .com.et (Ethiopian business) .org.al (Albania organization) .gov.it (Italian government) |
6. Second-Level Domains
A domain is a second-level domain if it is contained within a top-level domain. A second-level domain is a label – usually, a name related to the website or the business that owns it – immediately to the left of the top-level domain and separated by a dot.
| Full Domain | Second-Level Domain | Top-Level Domain |
|---|---|---|
| rjsnetworkcloudacademy.com | rjsnetworkcloudacademy | .com |
| google.co.uk | .co.uk | |
| github.io | github | .io |
| amazon.ca | amazon | .ca |
7. Subdomains
A subdomain – sometimes referred to as "third-level domains" – is related to the root domain and is denoted on the left as a second-level domain. In the URL "blog.rjsnetworkcloudacademy.com" the subdomain address would be "blog."
| Full Domain | Subdomain | Second-Level Domain | Purpose |
|---|---|---|---|
| blog.rjsnetworkcloudacademy.com | blog | rjsnetworkcloudacademy | Blog section |
| www.rjsnetworkcloudacademy.com | www | rjsnetworkcloudacademy | Main website |
| mail.google.com | Email service | ||
| docs.google.com | docs | Document service | |
| shop.amazon.com | shop | amazon | Shopping portal |
- Organize different sections of a website
- Host different services (blog, store, support)
- Target different geographic regions (us.example.com, uk.example.com)
- Separate development environments (dev.example.com, staging.example.com)
- No limit to the number of subdomains you can create
8. Hosts
The host part of an FQDN is used to identify an individual device – usually a server. In the FQDN "myserver.example.com" the hostname would be "myserver."
| FQDN | Host | Purpose |
|---|---|---|
| myserver.example.com | myserver | Generic server identification |
| webserver1.company.com | webserver1 | First web server |
| mailserver.domain.com | mailserver | Mail server |
| dbserver-prod.example.com | dbserver-prod | Production database server |
- Use descriptive names that indicate the server's function
- Include environment indicators (prod, dev, test)
- Add numbers for multiple instances (web1, web2, web3)
- Follow your organization's naming standards
- Keep names concise but meaningful
9. Practice Exercises
Exercise 1: Identify DNS Hierarchy Components
For the FQDN: mail.support.amazon.co.uk.
Identify:
- Root domain: . (dot)
- Top-Level Domain: .co.uk
- Second-Level Domain: amazon
- Subdomain: support
- Host: mail
Exercise 2: Create Valid Domain Names
Which of the following are valid domain names according to the LDH rule?
- ✅ example-site.com (Valid)
- ❌ -example.com (Invalid - starts with hyphen)
- ✅ my-web-site123.net (Valid)
- ❌ example_.com (Invalid - underscore not allowed)
- ✅ 123example.org (Valid)
- ❌ .123 (Invalid - TLD cannot be all numeric)
Exercise 3: DNS Resolution Order
For www.blog.company.com, DNS resolution proceeds in this order:
- Root DNS servers (.) – Direct to .com TLD servers
- TLD servers (.com) – Direct to company.com authoritative servers
- Authoritative servers (company.com) – Resolve blog.company.com
- Subdomain servers (blog.company.com) – Resolve www.blog.company.com
Exercise 4: Design Your DNS Hierarchy
Design a DNS hierarchy for a company called "TechCorp" with:
- Main website
- Blog section
- Online store
- Support portal
- Development environment
Suggested Structure:
- www.techcorp.com (main website)
- blog.techcorp.com (blog)
- shop.techcorp.com (online store)
- support.techcorp.com (support portal)
- dev.techcorp.com (development environment)
- DNS hierarchy is an inverted tree structure with five levels
- Domain names are interpreted from right to left (Root → TLD → SLD → Subdomain → Host)
- FQDN (Fully Qualified Domain Name) includes all components from root to host
- 13 root servers (A-M) handle the top level of DNS resolution
- TLDs can be organizational (.com, .org) or geographic (.uk, .ca)
- Second-level domains represent your brand or organization
- Subdomains organize different sections or services
- Labels must follow LDH rule (Letters, Digits, Hyphen)
- Maximum domain length is 255 characters including dots
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