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Setting the domain to "mysite.com" should work also with longer hostnames, provided that all of them end in "mysite.com".
Did you try it and find any problems?
Or is the "mysite.com" domain name...

I can't understand why sometimes you use host/domain names that end with ".com.au" and sometimes you use names that end with ".com".
May you also clarify which constraints your application server...

The setting of "domain names" on the pages is imposed by the browsers, any time you need to allow pages coming from different hosts to see each other.
See...