What is protocol 47?

Protocol 47, also known as GRE (Generic Routing Encapsulation), is a protocol used for encapsulating a wide variety of network layer protocols inside virtual point-to-point links or tunnels. It is commonly used for establishing a virtual private network (VPN) connection between two networks over the Internet.

GRE encapsulates packets by creating a tunnel between two endpoints, where the packets travel through the tunnel as if they were traveling directly between the two networks. This allows for secure communication between two networks that may be physically separated by a public network, such as the Internet, by essentially creating a private network within the public network.

GRE does not provide any encryption of the data that is being transmitted, so it is often used in conjunction with other protocols such as IPSec to provide additional security measures. Additionally, GRE has been used for multicast routing protocols such as PIM-SM or DVMRP for multicast traffic forwarding between routers.

Overall, GRE provides a flexible and efficient way of encapsulating multiple network protocols and allowing for secure, private communication between disparate networks.