DNS(Domain Name System) 구조The Structure of DNS (Domain Name System)

What Is DNS (Domain Name System)?

When we open Google in Chrome, we type google.com into the address bar. People usually call a text address like this a domain, but the precise term is a domain name.

To actually reach a specific server on the internet, you need a numeric IP address. Yet we can reach Google’s servers with just the domain name, without knowing Google’s IP address.

The system that turns human-friendly domain names into IP addresses computers can understand is called DNS (Domain Name System).

> DNS translates domain names into IP addresses

Looking Up an IP Address

Let’s look up the actual IP address of google.com. On Linux/Mac you can use the host command, and on Windows the nslookup command.

$ host google.com

google.com has address 172.217.31.174
google.com has IPv6 address 2404:6800:4004:820::200e
google.com mail is handled by 10 smtp.google.com.

> nslookup google.com

Name:    google.com
Addresses:  2404:6800:4004:820::200e
          172.217.31.174

The address of google.com that DNS gave us is 172.217.31.174. Try it and see whether you can connect using the IP address!

DNS Structure

So how did typing google.com into the address bar get us to the website? To understand that, we first need to know how DNS is structured.

DNS manages domain information in a hierarchy, much like a phone book. This hierarchical naming structure makes it possible to manage the countless domains around the world in an orderly way.

hierarchical naming structure

Let’s walk through it with a made-up address, http://api.www.google.com..

The DNS hierarchy is made up of four levels:

• Root Domain
• TLD(Top Level Domain)
• SLD(Second Level Domain)
• Sub Domain

Root Domain

Every domain name we use actually ends with a dot that stands for the Root Domain. For example, google.com is really the same as google.com., and that final dot represents the Root Domain.

> The domain names we use actually end with a dot

The Root Domain is the starting point of the DNS hierarchy and is written simply as a dot. In everyday use, though, that final dot is left out.

TLD(Top Level Domain)

In a domain name, dots act as separators between levels. Here, com, which comes after the next dot, is the TLD. The TLD is the level just below the Root Domain and means the top-level domain. There are two main kinds:

1. Generic top-level domains (gTLD): domains grouped by purpose, such as com, net, org
2. Country code top-level domains (ccTLD): domains that represent a country, such as kr, jp, us

> The TLD, one level below the Root Domain, is the top-level domain

SLD(Second Level Domain)

The SLD is the domain one level directly below the TLD. It’s usually the first level that individuals, organizations and companies register and manage themselves. Here, google is the SLD.

> The SLD is the domain one level directly below the TLD

The SLD is the most basic unit at which you can manage and configure DNS records yourself. The main DNS records are:

• A (Address Record) : DNS record for an IPv4 address
• AAAA (IPv6 Address Record) : DNS record for an IPv6 address
• CNAME (Canonical Name Record) : an alias for a domain
• NS (Name Server) : specifies the authoritative name servers that can provide the domain’s DNS information
• MX (Mail Exchange Record) : mail server settings

By configuring these DNS records, you can connect web servers, set up email services, redirect domains and more.

Sub Domain

A Sub Domain is any lower-level domain to the left of the SLD. It can be extended hierarchically as needed, and is mainly used to separate services or systems logically.

Each Sub Domain can have its own DNS records, so it can point to its own server or service independently. Common uses include:

• Separating services: docs.github.com, mail.google.com
• Separating environments: dev.example.com, staging.example.com, prod.example.com
• Separating regions: kr.example.com, us.example.com
• Separating functions: api.example.com, admin.example.com

> Sub Domain = every domain to the left of the SLD

Domain Name

With the DNS hierarchy covered, we can now understand what a domain name is. A domain name is an address that points to a specific location on the internet: in the dot-separated hierarchical address system, it means everything except the Root Domain.

> A domain name is the hierarchical address without the Root Domain

Each part is separated by a dot, and DNS reads a domain name from right to left when resolving it. For example, with api.www.google.com:

• Sub Domain: api, www
• SLD: google
• TLD: com

Thanks to this hierarchy, DNS can manage the world’s countless domain names in an orderly way and look them up efficiently.

FQDN (Fully Qualified Domain Name)

An FQDN is the full domain name including the Root Domain. The difference from a domain name is that it explicitly writes the Root Domain at the very end.

For example:

• Domain name : api.www.google.com
• FQDN : api.www.google.com.

> An FQDN is the full domain name including the Root Domain

An FQDN points to an exact domain location with no ambiguity, and it’s used whenever DNS settings need a precise domain. DNS configuration files and system settings in particular require FQDNs to specify the domain exactly.

Wrapping Up

We’ve covered the basic structure of DNS. DNS is built as the following hierarchy:

• The Root Domain is the starting point of the DNS hierarchy
• The TLD is the top-level domain, such as .com or .net
• The SLD is the domain you actually register and manage
• The Sub Domain is a lower-level domain you can add as needed

What’s Next

In the next post, How DNS (Domain Name System) Works, we’ll look in detail at how DNS turns a domain name into an IP address and how DNS servers carry out that process.