Cloud computing uses virtualization technologies to offer a non-shared use rental of computer resources with publicly accessible worker nodes. However, unlike Grid computing, Cloud computing as implemented by Amazon, IBM, Google and Microsoft only offers compute and storage resources from a single organization. Many of the cross-site and cross-organizational advantages offered by Grid computing are lost. In this paper, we present a novel infrastructure that combines the benefits of Grid and Cloud computing: Cheap multi-organizational resources and private compute nodes with root access reachable from the Internet. Our previously introduced virtualization of Grid resources is extended by an approach to offer the same freedom of network access Cloud computing offers, but in a multiorganizational and shared use environment without endangering existing users or resources. An approach is presented for the dynamic network isolation of Grid users from each other as well as a mechanism for shi...