In this paper, I focus on Secure VPNs. Just as there are an endless variety of physical network topologies, so too are there an infinite set of VPN topologies. At their core, however, most VPN topologies try to satisfy one of three applications. Intranet services can be offered to entire LANs or to a select set of authorized hosts on several LANs - for example, allowing accounting users to securely access a payroll server over a network segments that are not secured. In Extranet and B2B VPNs, a shared infrastructure and the associated soft provisioning of sites and users makes it possible to respond more rapidly and cost-effectively to changing business relationships.
Tunnels can exist at several protocol layers.
In a compulsory mode , an ISP's network access server intercepts a corporate user's PPP connections and tunnels these to the corporate network. In a voluntary mode , VPN tunnels extend all the way across the public network, from dial-up client to corporate network. Two layer 2 tunneling protocols are commonly used today. However, L2TP by itself does not provide message integrity or confidentiality. Layer 3 tunnels provide IP-based virtual connections.