Definition

What Is a Subdomain?

A prefix added to a root domain that creates a distinct web address — used to separate sections of a site like blog.reloadmode.com or app.saasproduct.com.

Related Terms

Custom Domain

DNS

Hosting

URL

SEO

Example

A SaaS company uses app.company.com for its product and help.company.com for its documentation, both as subdomains of company.com.

According to Ahrefs data, content on subdirectories (domain.com/blog) earns ranking benefits 3x faster than the same content on a subdomain (blog.domain.com) — because subdirectory content inherits the root domain's authority directly.

Synonyms:

Child domain, Prefix domain

Acronyms:

-

What it applies to

Situations where a part of a web presence is functionally distinct enough to justify a separate address — typically a product application, a documentation centre, or a localised site for a different market.

How to set up a subdomain

  • Go to your DNS provider (domain registrar or Cloudflare) and add a new record.
  • For a CNAME record: Name = "app", Value = the hostname of the hosting platform (e.g., your-app.vercel.app). This creates app.yourdomain.com.
  • For an A record: Name = "app", Value = the IP address of the server. Used when pointing to a specific server rather than a platform hostname.
  • Allow DNS propagation (minutes to 48 hours), then configure the platform to accept the subdomain as a custom domain.

Subdomain vs subdirectory — when to use each

  • Use a subdomain for: A separate product application (app.yourdomain.com), a completely different platform (help.yourdomain.com on Zendesk), or a distinct international site (fr.yourdomain.com).
  • Use a subdirectory for: A blog (yourdomain.com/blog), a resources section, a case studies library — any content that should contribute its SEO authority to the main domain.

Subdomain: Common Questions Answered

Question

Answer