An example tunnelling protocol
is to take an IP
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
then putting that
envelope in yet another envelope addressed to, say, your
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.