L2TP and PPTP were both designed for individual client to server connections; they enable only a single point-to-point connection per session. Dial-up VPNs use L2TP often. Both L2TP and PPTP operate at the data link layer (layer 2) of the OSI model. PPTP uses native PPP authentication and encryption services and L2TP is a combination of PPTP and Layer 2 Forwarding protocol (L2F).
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and Network Security (page 95).