An example tunnelling protocol is to take an IP packet and re-encapsulate it in yet another IP packet, then transmit it over the network to another computer which "detunnels" the packet by stripping off the first (or "outer") IP packet and then processing the remaining IP packet normally.

This is analagous to writing a letter to your mother and putting it into an envelope addressed to your mother, then putting that envelope in yet another envelope addressed to, say, your brother.

Drop the whole mess into the post, and (assuming your brother understands the protocol) your mother will get the message.

Uses for a tunnelling protocol are for when one needs to "extend" one part of a network into another part. Like IPX over IP, or virtual private networks, anonymous network services, and firewall traversal.

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