GRE Tunnel

The Generic routing encapsulation (GRE) is a communication protocol used to establish a direct, point-to-point connection between network nodes. Being a simple and effective method of transporting data over a public network, such as the Internet, GRE lets two peers share data they wouldn’t be able to share over the public network itself.

Two NB1600 connected through a GRE tunnel over the mobile interface.
NB1: 192.168.1.0|192.168.2.0
NB2: 192.168.3.0|192.168.4.0
Both have public IP adresses, 138.188.X.X

TUN mode:

Peer address: Public IP address of the remote station (NB1600-X, cisco router …)
Local tunnel address/netmask: Self defined GRE IP address (remote router local tunnel address: 192.168.8.2)
Remote network: Destination network / Network of the remote router

TAP mode:

Peer address: Public IP address of the remote station (NB1600-X, cisco router …)
Bridge interface: LANX; the IP address on both sides must be in the same subnet. (e.g local=192.168.1.1 / remote=192.168.1.2)