Is Addon domain affecting SEO

Question

I am just a learn in the field of SEO and i have a main domain and an addon domains. Both have separate websites. Consider main.com is my main domain and addon.com is my addon domain name which is pointed to a sub directory called "addon".

I can access addon.com by using the following 3 ways.

addon.com
main.com/addon
addon.main.com

Are these urls are indexed separately by search engines? If so how can i prevent this?

Does Search engine think main.com/addon as a page in the main.com?

I am not sure i need to worry about all these things or just leave it as it is. I searched to google but couldn't find a right answer.

Answer 1

It may be too late to answer. However, it may benefit others.

Primarydomain and subdomain or addon-domain will not be linked by the search engines automatically, unless you link them purposefully or inadvertently. Except all conditions are true:

1. Your web root normally public_html has no index page
2. Directory indexing of your web root is opened, eventually exposing/linking your sub-folder -which is attached to your addon-domain- to google and entire world.

In that scenario robots.txt solution is not recommended, because search engines may ignore robot.txt rules.

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