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Tunneling = allows a network user to access or provide a network service that the underlying network does not support or provide directly (wikipedia)

How does the "network service" not support a protocol? Is "network service" referring to a router, or a host? What problem does tunneling solve?

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What tunneling does is wrap a packet inside another packet. This could be used for something like allowing a protocol such as multicast to cross the public Internet, which doesn't support multicast, to another network which does support multicast. Simply wrap the multicast packet inside a unicast packet on one end, then strip off the unicast packet on the other end, leaving the original multicast packet.

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  • In you example: would the packet have to be forwarded to the right port (multicast) on the destination machine? How does it know to forward it to a different port? That seems like it might create a security risk too? – Kolob Canyon Jan 8 '18 at 17:37
  • You are confused. Ports have nothing to do with IP or IP packets. Ports are addresses for some transport protocols (TCP, UDP). – Ron Maupin Jan 8 '18 at 17:41
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    To clarify, a tunnel wraps the original packets in a new packet as the payload, and forwards that. As such, the intermediate network doesn't know and doesn't care what the payload is, as any other packet. The receiving device then dewraps the packet, and does whatever it does with the original packet. This means, as far as the tunnel is concerned, the payload could be the binary representation of pigeon droppings for all it cares. – Stuggi Jan 9 '18 at 14:08

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