DNS (Domain Name System)

DNS (Domain Name System) is the phonebook of the internet. It translates human-readable domain names like google.com into computer-readable IP addresses like 142.250.190.78.

Think of it like contacts on your phone:

• You search for “Mom” (domain name)
• Your phone looks up the number (IP address)
• It calls that number

Without DNS, you’d have to remember 142.250.190.78 instead of google.com.

A         → 142.250.190.78         # Maps domain to IPv4 address
AAAA      → 2001:db8::1            # Maps domain to IPv6 address
CNAME     → example.com            # Alias (one domain → another)
MX        → mail.example.com       # Mail server for the domain
NS        → ns1.example.com        # Name server for the domain
TXT       → "v=spf1 include:..."   # Text info (often for security)

Example of common records for example.com:

example.com.   A     93.184.216.34
example.com.   MX    mail.example.com
www.example.com. CNAME example.com

sequenceDiagram
    participant Browser as Your Browser
    participant Resolver as DNS Resolver<br/>(Your ISP)
    participant Root as Root Server
    participant TLD as TLD Server<br/>(.com)
    participant Auth as Authoritative<br/>Server

    Browser->>Resolver: What is the IP of example.com?
    Resolver->>Root: Where is the .com TLD server?
    Root-->>Resolver: Here's the .com server address
    Resolver->>TLD: Where is example.com?
    TLD-->>Resolver: Here's the nameserver for example.com
    Resolver->>Auth: What is the IP of example.com?
    Auth-->>Resolver: 93.184.216.34
    Resolver-->>Browser: IP is 93.184.216.34
    Browser->>Auth: HTTP GET example.com
    Auth-->>Browser: Here's the webpage!

Step by step:

1. Your browser asks the DNS Resolver (usually your ISP): “Where is example.com?”
2. The resolver asks a Root Server: “Where’s the .com server?”
3. Root responds: “Ask the .com TLD server at X.X.X.X”
4. Resolver asks the TLD Server (.com): “Where is example.com?”
5. TLD responds: “Ask the authoritative nameserver at Y.Y.Y.Y”
6. Resolver asks the Authoritative Server: “What’s example.com’s IP?”
7. Authoritative responds: “It’s 93.184.216.34”
8. Resolver tells the browser, and the browser connects

DNS results are cached (saved temporarily) at every level to make future lookups faster:

# Browser cache: seconds to minutes
# OS cache: minutes
# Router cache: hours
# ISP resolver cache: hours to days

Check your DNS cache (Windows):

ipconfig /displaydns

Check DNS resolution:

nslookup google.com
# Server:  dns.google
# Address:  8.8.8.8
# Name:    google.com
# Address:  142.250.190.78

# Or use dig (Linux/Mac)
dig google.com

# Find IP of a domain
nslookup github.com

# Find mail servers
nslookup -type=MX gmail.com

# Trace the full resolution path
dig +trace example.com

• DNS translates domain names (google.com) to IP addresses (142.250.190.78)
• It’s like a phonebook for the internet
• Resolution goes: Browser → Resolver → Root → TLD → Authoritative server
• DNS records include A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), TXT (text)
• Caching makes repeated lookups faster — at the browser, OS, router, and ISP levels