My "A" column is the network. Each cell in that column has the same
drop-down menu: NetworkA, NetworkB, etc.

My "B" column is the server name.

What I want is:

If NetworkB is selected for "A1", "B1" will have drop-down menu of list
of servers on NetworkB only.

If NetworkE is selected for "A2", "B2" will have drop-down menu of list
of servers on NetworkE only.

and so on...

How do I go about doing that?