eir private network beyond their immediate geographic area, one needs a fast and reliable communication network among their offices. The traditional VPN is using the leased lines on top of a wide area network (WAN). A WAN could very well be the public Internet because of its reliability, performance, and security. A private WAN can be expensive because leased lines can be expensive. More- over, leased lines are not viable for employees to access the corporate network wherever needed, for example, from home, from the road, or from other organizations. A VPN is an assembly of two or more private networks or individual users that uses secured tunnels over a public telecommunication infrastructure, such as the Internet, for connection. A VPN is com- monly used to provide distributed offices or individual users with secure access to their organization's network. VPN services enable remote access to the company Intranet at low cost, which enables the realization of mobile workforce. The VPN architecture supports a reliable authentication mechanism to provide easy access to the Intranet from anywhere anytime. The Intranet is accessible as long as the user has access to any edge media, including modems, ISDN, cable modems, DSL, fiber, Wi-Fi, and cellular wireless. The high-level architecture and process is shown in Figure 2.4. There are multiple types of VPN services, depending on its implementation. Link layer VPN: Link layer technologies such as frame relay and Asynchronous Transfer Mode (ATM) can be used to implement Customer Provider network network Encapsulated packet Figure 2.4 VPN illustration. 2 Virtualization and Cloud Computing the VPN if the underlying network is based on the link layer tech- nologies. The VPN is implemented using virtual circuits at the link layer. The frame relay frames or ATM cells are switched across nodes belonging to the virtual network. Virtual circuits are much cheaper than dedicated links and they are reconfigurable. The virtual circuits also have the advantage of providing SLAs. MPLS