GRE Tunnelling

Routers work by delivering and receiving packets of data, which are addressed using IP and Port numbers.

Tunnelling takes the principle of encapsulation that is already used when creating packets of data (wrapping up the data, headers and addressing information) and goes further by encapsulating the pack within another packet. By doing this, data is hidden from firewalls, meaning rules can be easily bypassed.

For GRE Tunnelling to work, the routers must support this format.