A network in which the routers can execute code that is contained in the packets passing through them. The packets may also point to the code with a URL or have an identifier for code previously loaded in the routers. Because the network's behavior can be altered at any time, active networks could be used to provide dynamic quality of service (QoS) or to support dynamic solutions to traffic congestion. However, because active networks can execute code, they would be a hacker's delight in a public venue such as the Internet. Thus far, active networks have only been developed for research environments. Contrast with passive network.