Comment by aftbit

My Google-fu is failing me right now, but there is a list of domains like dropboxusercontent.com that are treated as pseudo second-level domains for purposes like this.

e.g. u1234.dropboxusercontent.com is treated as a unique domain just like u1234-dropboxusercontent.com would be.

Edit: here we go, from another comment - the Public Suffix List: https://publicsuffix.org/