Cloud Computing facilitates the new generation of IT and the provisioning of services by introducing virtualization concepts throughout the network. Multi-tenancy introduces the capability to share cloud assets among service users. Service providers currently lack innovative means to enable true multi-tenancy in a cloud environment. There is an urgent need for technologies that introduce tenant security, traffic identification and tenant separation for better billing and fairness. This paper introduces a technology that tackles all these requirements by defining the concept of a Tenant-ID that is incorporated into the OSI Layer 2 frame header. We outline the concept of Tenant-IDs, the adapted 802.1q header and a way to automatically configure tenants via Cloud automation tools and software-defined networking approaches. The paper concludes by outlining interoperability to relevant layer-2 protocols and by introducing use-cases benefiting from the Tenant-ID concept.