In computer networking, Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs). It does not provide any encryption or confidentiality by itself; it relies on an encryption protocol that it passes within the tunnel to provide privacy.
Although L2TP acts like a Data Link Layer protocol in the OSI model, L2TP is in fact a Session Layer protocol, and uses the registered UDP port 1701. (see List of TCP and UDP port numbers).
(shamelessly copied from wikipedia)