L2TP, short for Layer Two (2) Tunneling Protocol, is a data link layer (layer 2 of the OSI Open Source Initiative model) protocol for tunneling network traffic between two peers over an existing network (usually the Internet), better known as VPN Virtual Private Networks. Because of the lack of confidentiality inherent in the L2TP protocol, it is often combined with IPsec Internet Protocol Security, which provides confidentiality, authentication, and integrity. The combination of these two protocols is also known as L2TP over IPsec. L2TP over IPsec allows you, while providing the same functions as PPTP, to give individual hosts access to your network through an encrypted IPsec tunnel.
Sophos UTM Administration Guide