Domain Name Contains Underscore. In this dialog box click on the change button. Provide here the name of the disjoint namespace in my example domainNamelab.
It is perfectly legal to have an underscore in a domain name. For a list of characters that can be be used view our guide to choosing a good domain name which also contains other handy tips and guidelines for registering au names or other extensions. However NETBIOS does support the underscore character as per your second link but DNS does not.
In this case blog is the sub-page.
The domain name does not actually include a period. Does Google Have a Preference. Do not use periods in new NetBIOS domain names. Provide here the name of the disjoint namespace in my example domainNamelab.