et is connected between selected users of the Internet and an intranet. These external users of a private intranet may be, for example, customers or mate- rial suppliers. Like an intranet, an extranet uses Internet technology, and for security reasons firewalls or other security gateway arrangements are used for user authentication purposes and data encryption. 2.10.4 INs A conventional telephone network is able to establish a connection only to a socket that is identified by the number of a B subscriber. There is no "intelli- gence" in this kind of operation; dialing a certain number makes every time a connection to a certain socket. Connection setup is always done in the same way, whether the intended B subscriber is available or not. In the old days, a human operator performed the switching process manually on a switchboard. If an operator knew that the called party was presently visiting his neighbor, she might connect the call directly to the neighbor's phone. There was some "intelligence" in the network that improved accessibility. In a modern telecommunications network this intelli- gence is implemented with help of IN technology. The IN is an ordinary digital telephone network with some additional capabilities like flexible routing of calls and voice notifications. Traditionally, a telephone number has been the identifier of a certain physical subscriber line and a socket. In an IN the physical number and service number have no fixed relation and may change with time. For example, emergency service may be available at daytime in multiple locations but at nighttime only in one location of the area. 2.10.4.1 Distributed Intelligence Network operators implement supplementary services, such as call forward- ing, to assist subscribers in making successful calls. This increases the number of successful calls, the utilization of the network, and, as a consequence, the network operator's revenue from call fees. We can implement these services by updating corresponding functions to each local exchange. Examples of