This chapter focused on traditional and modern VPN architectures. Traditional private networks are based on DLLs, Frame Relay, or ATM PVCs. Today, corporate customers can rent dark or lit fiber in addition to SDH circuits or single DWDM wavelengths. With the widespread introduction of IPSec and MPLS, various VPN offerings considerably changed.

This chapter also discussed tunnel solutions such as IP-IP, GRE, and user-space approaches in terms of transparency and application areas. Tunnels also play prominent roles connecting multicast or IPv6 islands over unicast and IPv4 transport networks. A thorough discussion of PPTP, L2F, and L2TP concluded the first part of this chapter. The second part was dedicated to user-space tunnels such as CIPE, V-TUN, and OpenVPN. The chapter concluded with a discussion of IPSec and general tunnel caveats.