We provide necessary and sufficient conditions for the node systems, the graph adjacency matrix, and the input matrix such that a heterogeneous network of multi-input multi-output linear time-invariant (LTI) node systems with constant linear couplings is controllable or stabilizable through the external input. We also provide specializations of these general conditions for homogeneous networks. Finally, we give a very simple, necessary and sufficient condition under which a homogeneous network of single-input single-output LTI node systems is stable in the absence of the external input.