L2TP allows the creation of a virtual private dialup network (VPDN)[5] to connect a remote client to its corporate network by using a shared infrastructure, which could be the Internet or a service provider's network.

https://en.wikipedia.org/wiki/Layer_2_Tunneling_Protocol



Introduction
A virtual private dial-up network (VPDN) allows a private network dial in service to span across to remote access servers (defined as the L2TP Access Concentrator [LAC]).

When a Point-to-Point Protocol (PPP) client dials into a LAC, the LAC determines that it should forward that PPP session on to an L2TP Network Server (LNS) for that client. The LNS then authenticates the user and starts the PPP negotiation. Once PPP setup has completed, all frames are sent through the LAC to the client and the LNS.

https://www.cisco.com/c/en/us/support/docs/dial-access/virtual-private-dialup-network-vpdn/20980-vpdn-20980.html