What is the proper way to delegate to a private / hidden sub-domain?

[John Levine]

In article <mailman.369.1588801956.942.bind-users at lists.isc.org> you write:
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>
>On 5/6/20 3:40 PM, John Levine wrote:
>> Can clients on the internal network contact hosts in the outside 
>> world, or is it really disconnected?
>It depends on which particular lab is being used and what is being tested.
>
>I can guarantee that some of the labs will NOT have access to other 
>networks, much less the Internet.  (Some of them even have a protocol 
>barrier.)

Since they can't access the root servers, how do you expect them to
do DNS lookups at all?

R's,
John