Subdomain
A label prefixed to a domain, like 'shop' in shop.example.com. Used to delegate sections of a domain to different services or tenants.
A subdomain is anything to the left of the base domain. shop.example.com is a subdomain of example.com. api.v2.example.com is a multi-level subdomain.
Subdomain vs subfolder
Two ways to organize a section of your site:
| Approach | URL | Tech |
|---|---|---|
| Subdomain | blog.acme.com | DNS record |
| Subfolder | acme.com/blog | App routing |
Functionally identical for users. SEO debates rage about which is better; consensus in 2026 is that for most SaaS, subfolders win because authority signals consolidate to the apex domain. Exceptions exist (separate brand, separate locale).
Why SaaS uses subdomains
Multi-tenant SaaS often uses subdomains for tenant isolation:
acme.yourapp.com → tenant: acme
shop.yourapp.com → tenant: shop
One wildcard DNS record (*.yourapp.com) plus a wildcard SSL cert handles all of them. Cheap to operate.
Tenants who want branding eventually upgrade to a custom domain on their own root (hello.acme.com).
Limits
- A subdomain can have its own CNAME, TXT, MX, etc.
- A subdomain can be delegated to a different DNS provider via NS records.
- Wildcard SSL certs cover ONE level of subdomain.
*.example.comcoversa.example.com, nota.b.example.com. - DNS lookups for deep subdomains add latency unless cached.