Introduction to Tunneling 525
Among the above tunnels, the IPv6 manual tunnel is a configured tunnel, while
the 6to4 tunnel, and intra-site automatic tunnel address protocol (ISATAP) tunnel
are automatic tunnels.
1 IPv6 manually configured tunnel
A manually configured tunnel is a point-to-point link. One link is a separate
tunnel. The IPv6 manually configured tunnels provide stable connections requiring
regular secure communication between two border routers or between a border
router and a host for access to remote IPv6 networks.
2 6to4 tunnel
An automatic 6to4 tunnel is a point-to-multipoint tunnel and is used to connect
multiple isolated IPv6 networks over an IPv4 network to remote IPv6 networks.
The embedded IPv4 address in an IPv6 address is used to automatically acquire the
destination of the tunnel. The automatic 6to4 tunnel adopts 6to4 addresses. The
address format is 2002:abcd:efgh:subnet number::interface ID/64, where
abcd:efgh represents the 32-bit source IPv4 address of the 6to4 tunnel, in
hexadecimal notation. For example, 1.1.1.1 can be represented by 0101:0101.
The tunnel destination is automatically determined by the embedded IPv4 address,
which makes it easy to create a 6to4 tunnel.
Since the 16-bit subnet number of the 64-bit address prefix in 6to4 addresses can
be customized and the first 48 bits in the address prefix are fixed by a permanent
value and the IPv4 address of the tunnel source or destination, it is possible that
IPv6 packets can be forwarded by the tunnel.
3 ISATAP tunnel
With the application of the IPv6 technology, there will be more and more IPv6
hosts in the existing IPv4 network. The ISATAP tunneling technology provides a
satisfactory solution for IPv6 application. An ISATAP tunnel is a point-to-point
automatic tunnel. The destination of a tunnel can automatically be acquired from
the embedded IPv4 address in the destination address of an IPv6 packet. When an
ISATAP tunnel is used, the destination address of an IPv6 packet and the IPv6
address of a tunnel interface both adopt special addresses: ISATAP addresses. The
ISATAP address format is prefix(64bit):0:5EFE:ip-address. The ip-address is in the
form of a.b.c.d or abcd:efgh, where abcd:efgh represents a 32-bit source IPv4
address. Through the embedded IPv4 address, an ISATAP tunnel can automatically
be created to transfer IPv6 packets. The ISATAP tunnel is mainly used for
connection between IPv6 routers or between a host and an IPv6 router over an
IPv4 network.
Figure 158 Principle of ISATAP tunnel
IPv6 network
IPv4 network
ISATAP tunnel
IPv6 host
IPv4/IPv6 host
ISATAP router
IPv4 address:
2.1.1.1/24
IPv6 address:
FE80 ::5EFE:0201:0101
3FFE::5EFE:0201:0101
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