What is a subdomain?

Subdomains let you carve a domain into named sections without registering separate domains. app.example.com, api.example.com, and docs.example.com can all be configured to point to different servers, run different stacks, and have independent SSL certificates, while sharing a single registered domain.

DNS-wise, a subdomain is just an A or CNAME record under the parent zone. There's no limit on how many subdomains you can create, and you can nest them (v2.api.example.com). Each level becomes a separate origin from the browser's perspective, with implications for cookies, CORS, and storage.

For SEO, subdomains historically were treated as separate sites by Google. That's softened over the years (modern Google generally treats them as part of the same brand) but you still see consultants debate whether blog.example.com or example.com/blog is better for rankings. The honest answer is that either works; pick the one that's easier to operate.