Abstract: In a packet based (typically IP) network, network address translation is frequently used to overcome the problem of dwindling address spaces. However, network address translation prevents the operation of many of the complex protocols used for transmission of media streams for example, data such as voice over IP or video over IP. Accordingly, media proxy devices are used to allow such data to traverse NAT boundaries. However, media proxy devices are expensive and therefore scarce and the selection of an optimum media proxy is important to obtain optimum functionality and cost effectiveness within the network. By providing a call agent in the network with a simple model of the network and providing information concerning optimum media proxies for different parts of the network, the problem of selecting an optimum media proxy during path set up is overcome.