Subdomain

What is a Subdomain?

A subdomain is a domain that exists as part of a larger parent domain, created by adding a prefix to the main domain name. In "blog.example.com," "blog" is the subdomain of "example.com." Subdomains allow website owners to organize content, host separate applications, or create distinct sections without registering additional domain names. They're created through DNS configuration at no extra registration cost.

Subdomain Structure

blog.example.com
  │      │     │
  │      │     └── TLD (Top-Level Domain)
  │      └── Root domain (Second-Level Domain)
  └── Subdomain (Third-Level Domain)

Multiple levels possible:
dev.api.example.com
 │   │      │
 │   │      └── Root domain
 │   └── First subdomain
 └── Second subdomain (fourth level)

Common Subdomain Uses

Creating Subdomains

DNS Configuration

Add records for the subdomain in your DNS zone:

; A record pointing to IP
blog.example.com.    A     192.0.2.10

; CNAME pointing to another domain
shop.example.com.    CNAME  mystore.shopify.com.

; Multiple subdomains
api.example.com.     A     192.0.2.20
dev.example.com.     A     192.0.2.30

Wildcard Subdomains

Match all undefined subdomains:

*.example.com.    A    192.0.2.100

Subdomain vs Subdirectory

When to Use Subdomains

• Different platforms/technologies
• Separate applications
• Multi-tenant systems
• Geographic targeting
• Development environments

When to Use Subdirectories

• Blog or content sections
• SEO consolidation desired
• Simple site organization
• Same technology stack

Subdomain SEO Considerations

Search Engine Treatment

• Google treats subdomains as separate sites
• Link equity doesn't automatically transfer
• Each subdomain builds authority independently
• May require separate Search Console verification

Best Practices

1. Use subdomains for truly separate entities

2. Use subdirectories to consolidate SEO

3. Implement proper canonicalization

4. Set up separate analytics tracking

5. Create XML sitemaps for each subdomain

Security Considerations

SSL Certificates

Cookie Scope

Set-Cookie: session=abc; Domain=.example.com
# Accessible to all subdomains

Set-Cookie: session=abc; Domain=blog.example.com
# Only accessible to blog subdomain

Subdomains provide flexible organization for websites and applications, allowing distinct sections to operate independently while remaining part of the parent domain's namespace.