  We propose the concept of open network as an arbitrary selection of nodes of
a large unknown network. Using the hypothesis that information of the whole
network structure can be extrapolated from an arbitrary set of its nodes, we
use Renyi mutual entropies in different q-orders to establish the minimum
critical size of a random set of nodes that represents reliably the information
of the main network structure. We also identify the clusters of nodes
responsible for the structure of their containing network.
