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How allowlisting works on subdomains

A guide to help publishers and their visitors navigate how to allowlist properties with subdomains.


Publishers with properties that have subdomains should be aware of certain nuances regarding allowlisting.  This information is most pertinent to properties that have heavy visitor cross-traffic between their root domain and subdomains. 


1. How Subdomain Allowlisting Works
When a visitor allowlists a subdomain, it includes that subdomain and any subdomains created beneath it. This is because allowlisting works upward, away from the root domain. 

Example: A visitor who allowlists a.com. automatically allowlists the subdomain b.a.com and the sub-subdomain c.b.a.com. 


2. Impact on Root Domains
Allowlisting a subdomain does not automatically extend the allowlist to the root domain. It remains specific to the subdomain and does not affect the root domain. This is because allowlisting does not work downward, towards the root domain.

Example: A visitor who allowlists the subdomain b.a.com or sub-subdomain c.b.a.com does not automatically allowlist the root domain a.com. 

3. Impact on Other Subdomains
Allowlisting a subdomain includes that subdomain and any subsequent subdomains beneath it. It does not automatically allowlist other subdomains at the same level or higher in the hierarchy (i.e. upward toward the root).

Example: A visitor who allowlists b.a.com automatically allowlists c.b.a.com, but does not automatically allowlist subdomains of the same hierarchy, such as z.a.com.