Juan,
We don't have any restriction to the sub-portals domain.
Which version of dotnetnuke are you using?
Have you got it works with your main site?
When you talk about the subportal? Do you mean the following structure?
abc.com
abc.com/portal1
abc.com/portal2
or
abc.com
domain1.abc.com
domain2.abc.com
Keep me posted.
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