Network virtualization 275 Virtualized networks Network virtualization is a mechanism used to separate a single physical network into multiple logical networks. Virtual networks are often referred to as "slices." Each slice can be configured with different protocols and thus can be optimized for a different application domain. Figure 15-6 illustrates how two slices with different protocol stacks share a single network infrastructure. The virtual topology of each slice is also shown. It is important to note that in the context of next-generation networks, virtualiza- tion refers to logical network configurations that span the entire protocol stack from the link layer to the application layer. This type of virtualization should not be con- fused with virtualization technology used in local area networks (LANs). Virtual LANs (VLANs) are used to configure logical subnets within a single physical local area network [65]. In such VLANs, the protocol stack is typically the same for all virtual networks. Thus, VLAN technology does not provide as comprehensive a virtualization solution as is necessary for next-generation networks. However, VLANs can be used as a partial solution for the link layer as discussed later.