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VPN hardware, which includes VPN servers, clients, and other hardware devices, such as VPN routers, gateways, and concentrators

VPN software, which includes server and client software and VPN management tools

Security infrastructure of the organization, which typically includes RADIUS, TACACS, NAT, and AAA-based solutions

Service provider’s supporting infrastructure, which includes the service provider’s network access switching backbone and the Internet backbone

Public networks, which include the Internet, Public Switched Telephone Networks (PSTNs), and Plain Old Telephone Services (POTS)

Tunnels, which might be PPTP-based, L2TP-based, or based on L2F

From Building a Virtual Private Network By Meeta Gupta. Available from Safari Books Online.