System Design Fundamentals

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Introduction to DNS

On This Page

What DNS Is

Why DNS Matters

Reading a Domain Name

The DNS Hierarchy

Who Runs Each Level

DNS Records

DNS Resolvers

One Lookup, End to End

Key Takeaways

Practice Questions

When you call a friend, you tap their name in your contacts. You do not type their phone number from memory. The phone looks up the number for you.

The internet works the same way. You type www.example.com, but computers cannot connect to a name. They connect to an IP address, a number like 203.0.113.10.

So something must turn the name into the number, quickly, for billions of names. This lesson explains what that system is, how it is organized, and who does the work of each lookup.

What DNS Is

The Domain Name System (DNS) translates human-readable domain names, like www.designgurus.io, into the IP addresses that computers use to communicate.

DNS works like a phonebook for the internet. You look up a name, and you get back the number you actually need to make the call. People remember names, and computers use the numbers.

An IP address identifies a machine on a network. There are two versions in use.

IPv4 addresses have four numbers separated by dots, like 203.0.113.10.
IPv6 addresses are longer and are written in hexadecimal with colons, like 3fff::10.

DNS answers are small, so most DNS queries use UDP on port 53. DNS switches to TCP when a response is too large for UDP. The TCP vs. UDP lesson explains the two protocols.

A domain name goes into DNS, which looks it up and returns the IP address that the computer needs

Why DNS Matters

Without DNS, people would have to memorize numbers to use the internet. DNS gives four main benefits.

User-friendliness. Domain names are easier to remember and type than IP addresses, which are long strings of digits.
Scalability. DNS is a distributed and hierarchical system, not one giant server. The work and the data are spread across many servers, so DNS can handle the huge and growing number of names on the internet.
Flexibility. A website can change its IP address without affecting users. The DNS records are updated, and people keep reaching the site through the same domain name.
Load balancing. DNS can spread user requests across several servers, which improves the performance and reliability of a website. The DNS load balancing lesson covers how.

Reading a Domain Name

A domain name is a human-readable address made of labels separated by dots, like blog.example.com. Read it from right to left because the most general part comes last.

The root. Every full domain name really ends with a dot, like blog.example.com., although we usually leave the dot out. That final dot stands for the root of DNS.
Top-level domain (TLD). The TLD is the rightmost visible part, like .com in www.example.com.
Second-level domain. The part just left of the TLD, like example. Together, example.com is the domain that an organization registers.
Subdomain. A subdivision of a domain, used to create a separate section of a site. It appears to the left of the main domain. In blog.example.com, blog is a subdomain of example.com.

TLDs come in two main kinds.

Generic TLDs (gTLDs) name a general category, like .com, .org, and .net.
Country-code TLDs (ccTLDs) represent a country or territory, like .us for the United States or .uk for the United Kingdom.

A name that includes every part, down to the root, is called a fully qualified domain name (FQDN), like blog.example.com.

Read from right to left, blog.example.com. has the root, the top-level domain, the second-level domain, and a subdomain

The DNS Hierarchy

DNS can handle so many names because it is split into levels. No single server knows every name. Instead, three kinds of servers each know one level, and each points to the next.

Root servers are at the highest level. They direct queries to the right TLD servers. For example, a root server does not know example.com, but it knows which servers handle .com.

There are 13 root server clusters worldwide, named with the letters A to M, and run by 12 different organizations. Each cluster is made of many physical servers in many countries, for redundancy and reliability.

TLD servers store information about the domain names inside their own TLD. For example, the .com TLD servers do not store the IP address of example.com. They store which name servers are responsible for it, and they point the query there.

Authoritative name servers hold the actual DNS records for a domain, including its IP address. They give the final answer to a DNS query.

Root servers point to TLD servers, TLD servers point to the authoritative servers for a domain, and the authoritative servers hold its records

Only the authoritative server knows the answer. The root and TLD servers only point the resolver to the next level, one step at a time.

Who Runs Each Level

A registry runs the servers for a TLD. For example, one company runs the .com TLD servers.
A registrar is a company where you buy a domain name. When you register example.com, the registrar tells the .com registry which name servers are authoritative for your domain.
The authoritative name servers are often run by a DNS hosting provider, a cloud provider, or the registrar itself.

The set of DNS records that one organization manages, like example.com and its subdomains, is called a zone.

DNS Records

An authoritative server stores its data as DNS records. Each record has a name, a type, a value, and a TTL (time to live). The TTL says for how many seconds other servers may cache the record.

Here are a few records for example.com, in the text format that DNS servers use.

example.com.       3600  IN  A      203.0.113.10
example.com.       3600  IN  AAAA   3fff::10
www.example.com.   3600  IN  CNAME  example.com.
example.com.       3600  IN  MX     10 mail.example.com.
example.com.       86400 IN  NS     ns1.dnshost.net.

The number 3600 is the TTL, which here is one hour. IN means internet. The most common record types are below.

Type	What it stores	Example use
A	An IPv4 address	`example.com` is at `203.0.113.10`
AAAA	An IPv6 address	`example.com` is at `3fff::10`
CNAME	Another name, as an alias	`www.example.com` is the same as `example.com`
MX	The mail server for the domain	Email for `example.com` goes to `mail.example.com`
NS	The authoritative name servers	`example.com` is managed by `ns1.dnshost.net`
TXT	Any text	Proof that you own the domain, or email security settings

When a browser needs the IP address of www.example.com, it asks for an A or AAAA record. If it finds a CNAME instead, the lookup continues with the name that the CNAME points to.

DNS Resolvers

A DNS resolver is any software or server that turns a domain name into an IP address. There are several kinds, and they differ in how much of the work they do.

Stub resolver. This is the small DNS client on your device. It knows one or more DNS servers to ask. It sends the query to one of them, waits for the response, and returns the result to the application, like a web browser. It does not query the root, TLD, and authoritative servers itself.

Your router usually gives your device the DNS server to use. You can also set one by hand, like 8.8.8.8 for Google Public DNS or 1.1.1.1 for Cloudflare.

Recursive resolver. This DNS server does the full lookup for the client. It checks its own cache first. If the answer is not there, it queries the root servers, then the TLD servers, then the authoritative servers, until it finds the answer. Then it caches the result and returns it. Internet providers run recursive resolvers, and Google Public DNS, Cloudflare, and OpenDNS are public examples.

Caching-only resolver. Its main job is to cache the results of DNS queries and reuse them to speed up later lookups. It hosts no DNS zones of its own, so it is not authoritative for any domain. Many home routers work this way. After one device in a home looks up a site, the next device gets the answer from the router almost at once.

Forwarder. This DNS server passes queries it cannot answer to another DNS server upstream, instead of doing the full lookup itself. It may still keep a local cache. Corporate networks often use forwarders, so they can manage, log, or filter DNS in one central place.

Iterative (non-recursive) resolver. It does not look for the answer elsewhere. It returns what it already knows, or a referral, which is the name of another server to ask next. Authoritative servers usually work this way. When asked about a subdomain they do not handle, they reply with a referral to the name server that does.

The stub resolver only asks, the recursive resolver does the whole lookup, the caching-only resolver reuses answers, the forwarder passes queries upstream, and the iterative resolver returns a referral

One Lookup, End to End

Here is how the pieces work together when you visit www.example.com for the first time.

You type the name into your browser.
Your device's stub resolver sends one query to its configured DNS server, like your internet provider's resolver or a public one.
That recursive resolver checks its cache. If the answer is there, it replies at once, and the lookup ends here.
If not, the resolver asks a root server, which points it to the .com TLD servers.
The resolver asks a .com TLD server, which points it to the authoritative servers for example.com.
The resolver asks an authoritative server, which returns the IP address.
The resolver caches the answer for its TTL and returns it to your device.
Your browser connects to that IP address, and the page starts to load.

A lookup goes from the browser to the stub resolver and the recursive resolver, which uses its cache or asks the root, TLD, and authoritative servers before returning the address

Step 3 matters most in practice. Popular names are almost always in the resolver's cache, so most lookups end there, in a few milliseconds. The next lesson explains why.

Key Takeaways

DNS translates human-readable domain names into IP addresses, like a phonebook for the internet.
DNS is user-friendly, scalable because it is distributed and hierarchical, flexible because a site can change its IP address, and it can balance load.
Read a domain name from right to left: root, TLD, second-level domain, then subdomains. TLDs are generic, like .com, or country codes, like .uk.
Root servers point to TLD servers, TLD servers point to authoritative servers, and authoritative servers hold the records. There are 13 root server clusters, each with many servers.
DNS records have types, like A, AAAA, CNAME, MX, NS, and TXT, and each record has a TTL.
A stub resolver only asks. A recursive resolver does the whole lookup. A caching-only resolver reuses answers, a forwarder passes queries upstream, and an iterative resolver returns referrals.

DNS turns a name that people can remember into an address that computers can use, and no single server has to know every name. The next lesson, DNS Resolution Process, looks closely at how queries travel and how caching keeps most lookups fast.

Practice Questions

Try each question first, then open the answer.

1. In the name docs.api.example.org, which part is the TLD, which part is the registered domain, and which parts are subdomains?