Editing Point-to-Point Protocol (section)

=== As a layer 2 protocol between both ends of a tunnel ===

Many protocols can be used to [[Tunneling protocol|tunnel]] data over IP networks. Some of them, like [[Secure Sockets Layer|SSL]], [[Secure Shell|SSH]], or [[L2TP]] create [[virtual network interface]]s and give the impression of direct physical connections between the tunnel endpoints. On a [[Linux]] host for example, these interfaces would be called '''tun0''' or '''ppp0'''.

As there are only two endpoints on a tunnel, the tunnel is a point-to-point connection and PPP is a natural choice as a data link layer protocol between the virtual network interfaces. PPP can assign IP addresses to these virtual interfaces, and these IP addresses can be used, for example, to route between the networks on both sides of the tunnel.

[[IPsec]] in tunneling mode does not create virtual physical interfaces at the end of the tunnel, since the tunnel is handled directly by the TCP/IP stack. [[L2TP]] can be used to provide these interfaces, this technique is called L2TP/IPsec. In this case too, PPP provides IP addresses to the extremities of the tunnel.