Is there a way to tell a search engine that a subdomain should be treated as part of your main domain?

Question

I've heard that subdomains are bad for SEO (e.g. this and many others), but I don't see any technical reason that should be so.

I'm curious if there's programmatic a way to tell a search engine (like Google) that it should treat your subdomain as part of your main domain? I'm currently using JSON-LD to mark up our pages for other SEO purposes.

A related question is that I've also heard Google doesn't care whether it's a subdomain or not, which also makes sense. But certainly there are cases where subdomains are pretty much unaffiliated with the main domain (e.g. wordpress.com vs subdomains of that owned by individual users). Is there a programmatic way for wordpress.com to tell a search engine it wants to be distanced from their subdomains for SEO purposes?

Answer 1

Public Suffix List

In cases where you are giving out subdomains to users, and you don’t necessarily trust each other (e.g., being a web hoster), you can get your domain added to the Public Suffix List.

This list can be used by various parties. It could help browsers to improve security (e.g., cookie isolation) and usability (e.g., better grouping/sorting in the interface). It could help services to apply better rate limiting (e.g., for generating Let’s Encrypt certificates). Etc.

But the Public Suffix List has no SEO relevance (it might be used by search engines, but there is no reason to do anything with it for SEO).

SEO

Search engines could implicitly determine that documents on different subdomains "belong together": these documents would be linked.

If you have a site under example.com and a corresponding blog under blog.example.com, both should link each other in a navigation section, and so the connection is established.

If you have a blog hosting service under example.com and give out blogs under alice.example.com, john.example.com etc., the sites would typically not link each other (or only on a few pages in the content section), and so there is no strong connection.

There is nothing you have to do for SEO, it just comes naturally by linking where it makes sense, no matter whether the documents are on the same subdomain, different subdomains, or totally different domains.