by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568 Pub Date: 11/01/98
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Illustrated TCP/IP
by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568 Pub Date: 11/01/98 |
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Now, which type of tunneling is used where, and how does this thing work? Well, it depends on the address of the destination. If the address is an IPv6 address and the destination is local (on-link), then it simply sends the packet. If the address of the end node is an IPv4 address and it resides on a different subnet, then an IPv4 router must be used.
The key to all of this is the special address 0:0:0:0:0:0:<IPv4 32-bit address>. IPv4-compatible router.
Dual IP stack hosts will recognize the special address and immediately encapsulate the packet with an IPv4 header. This is called an end-to-end tunnel. The receiving station will decapsulate the datagram (strip off the IPv4 header) and read it as an IPv6 datagram.
IPv6-only hosts can also use the IPv4 Internet through the use of dual-stack IP routers. The IPv6-only host will transmit the IP datagram as an IPv6 datagram. The dual-stack IP router will recognize the special address and wrap it in an IPv4 header (using the last 32 bits of the special address in the IPv4 destination IP address).
Finally, if the address is an IPv6 address but not of the special address, a configured tunnel can be used instead of an automatic tunnel (which recognizes the special address). This requires configuration in the IPv6 host to allow for this. This is a more or less manual configuration and tells the IPv6 host where to send the packet.
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