he sys- tems in the network are the property of the network operator. In effect, a VPN provides a dedicated network for the customer with the help of public network equipment. As companies concentrate more and more on their core businesses, they are willing to outsource the provision, management, and maintenance of their telecommunications services to a public network operator that has skilled professionals dedicated to telecommunications. The principle of VPN is used for voice services such as corporate PBX/PABX networks. In this case the network that interconnects the offices of a company uses (voice or 56/64 Kbps) channels from the public network that are leased from a public network operator. An important application of VPN is intranet use. An intranet is a pri- vate data network that uses open Internet technology. Physically, an intranet may be made up of many LANs at different sites. To interconnect these LANs, a VPN is established to provide data transmission between sites through the public Internet network. Note that the Internet uses the packet- switching principle and there are no physically separate channels for each VPN as in the previously explained voice VPN. Because the packets are not separated into dedicated point-to-point channels, security risks arise when the public Internet is used for interconnections instead of leased lines or a circuit-switched network such as ISDN. To overcome this problem, firewalls are used in an intranet at the interface between each LAN and the public Internet. The firewalls perform the authentication duties for the communi- cating parties and they encrypt and encapsulate data for transmission through the public Internet from one office to another. A dedicated secure data pipe through the Internet is established with the help of encapsulation The Telecommunications Network: An Overview and ciphering and then the Internet can be used instead of a more expensive leased or circuit-switched data connection. Another network related to an intranet is an extranet. An extran