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Virtualization is the cornerstone of modern private and public cloud solutions. By enabling the consolidation of many virtual machines (VMs) within a physical server, virtualization blurs the frontier between networking and system. The interconnection network now starts within the physical servers, where several (virtual) servers compete to access the network interface cards (NICs). However, our understanding of the networking service offered to VMs by a specific hypervisor is largely unknown. In this work, we adopt an experimental approach to uncover different costs related to accessing a virtual NIC (as exposed by the hypervisor to the VMs), sharing it among VMs and to managing the VMs themselves, which potentially introduce an extra delay at each packet emission. We consider the case of Xen and VMware and propose a technique to identify those different delays through the design of specific experimental scenarios.