A virtualization device for composing a virtual network for implementation using physical network infrastructure, the virtualization device comprising; a node functionality and constraints module, configured to identify functionality and constraints of one or more nodes of the physical network infrastructure; a link constraints module, configured to identify constraints of one or more links of the physical network infrastructure; a network modeller module, configured to generate a model of the physical network infrastructure, based on the identified functionality and constraints of the nodes and on the identified constraints of the links; a virtual network composer module configured to receive a request for composition of a desired virtual network and to map the desired virtual network onto the physical network infrastructure based on the model generated by the network modelling module; and one or more node and link mapping modules for simultaneously mapping a virtual node of the desired virtual network onto one or more physical nodes or part of a physical node of the physical network infrastructure and for mapping a virtual link associated with the virtual node onto one or more physical links or part of a physical link of the physical network infrastructure.