Networking infrastructure is a vital part of virtual computer clusters. This paper describes VirtCloud, a system for interconnecting virtual clusters in a state-wide network based on advanced features available in academic networks. The system supports dynamic creation of virtual clusters without the need of run-time administrative privileges on the backbone core network, encapsulation of the clusters, controlled access to external sources for cluster hosts, full user access to the clusters, and optional publishing of the clusters. The paper describes architecture of the system, and prototype implementation in MetaCenter (Czech national Grid infrastructure) using Czech national research network CESNET2. Feasibility of the concept is evaluated through a series of measurements demonstrating that the network performance of the system is satisfactory.