When setting up a subdomain like blog.example.com or shop.example.com, you'll need to create a DNS record that tells the internet how to reach it. The two most common record types for this purpose are A and CNAME.

Use an A (Address) record when your subdomain points directly to an IP address:

blog.example.com.    IN  A    192.0.2.1

Key characteristics:

• Direct mapping to IPv4 address
• Best for when you control the server IP
• Faster resolution (no extra lookup)
• Common for root domains and static IP subdomains

Use a CNAME (Canonical Name) record when your subdomain should point to another domain name:

help.example.com.    IN  CNAME    help.zendesk.com.

Key advantages:

• Points to another domain (useful for third-party services)
• Automatically updates when target IP changes
• Great for cloud services with changing IPs
• Common for CDNs or SaaS integrations

Important rules to remember:

# Example of what NOT to do:
example.com.    IN  CNAME    othersite.com.  ; WRONG for root domain

• Never use CNAME for root domain (@ or example.com)
• A records can't point to domain names, only IPs
• CNAME can't coexist with other records for same name
• TTL (Time-To-Live) affects how quickly changes propagate

For a Node.js app running on a subdomain:

// If using direct server (A record)
dev.example.com.    IN  A    203.0.113.45

// If using Heroku (CNAME record)
staging.example.com.    IN  CNAME    quiet-rain-1234.herokudns.com.

After setting up your record:

1. Use dig subdomain.example.com to verify
2. Check propagation with nslookup
3. Remember DNS changes can take up to 48 hours
4. Clear local DNS cache if needed

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When setting up a subdomain (e.g., blog.yourdomain.com), you need to configure DNS records to point it to the correct server. The two most common record types for this purpose are A and CNAME records.

A Record (Address Record):
Maps a domain/subdomain directly to an IP address.

example.com.    A     192.0.2.1
blog.example.com. A   192.0.2.1

CNAME Record (Canonical Name):
Aliases one name to another. Useful when pointing to another domain name rather than IP.

blog.example.com. CNAME example.com.

Use A record when:
- You know the exact IP address of your server
- The IP address won't change frequently
- You need to point directly to infrastructure

Use CNAME record when:
- You're pointing to another domain name (e.g., AWS, GitHub Pages)
- The IP might change often (cloud services)
- You want to delegate DNS management

Example 1: Basic A Record Setup

; Zone file excerpt for example.com
@        IN  A      192.0.2.1
www      IN  A      192.0.2.1
blog     IN  A      203.0.113.5

Example 2: CNAME for Cloud Service

; Pointing to GitHub Pages
blog     IN  CNAME  yourusername.github.io.

After setting up records:
- Use dig or nslookup to verify
- Consider TTL (Time-To-Live) values for changes
- Remember DNS propagation delays (typically 24-48 hours)

Dig command example:

dig blog.example.com +nostats +nocomments +nocmd

1. DNS Propagation: Changes aren't immediate
2. CNAME Chains: Avoid multiple CNAME redirects
3. Root Domain CNAME: Never use CNAME for root domain (@)