ISATAP is an automatic overlay tunneling mechanism that uses the underlying IPv4 network as a NBMA link layer for IPv6. ISATAP
is designed for transporting IPv6 packets
within a site where a native IPv6 infrastructure is not yet available; for example, when sparse IPv6 hosts are deployed for testing.
ISATAP tunnels allow individual IPv4 or IPv6 dual-stack hosts within a site to communicate with other such hosts on the same
virtual link, basically creating an IPv6 network using the IPv4 infrastructure.
The ISATAP router provides standard router advertisement network configuration support for the ISATAP site. This feature
allows clients to automatically configure themselves as they would do if they were connected to a GigabitEthernet or FastEthernet.
It can also be configured to provide connectivity out of the site. ISATAP uses a well-defined IPv6 address format composed
of any unicast IPv6 prefix (/64), which can be link local, or global (including 6to4 prefixes), enabling IPv6 routing locally
or on the Internet. The IPv4 address is encoded in the last 32 bits of the IPv6 address, enabling automatic IPv6-in-IPv4 tunneling.
Although the ISATAP tunneling mechanism is similar to other automatic tunneling mechanisms, such as IPv6 6to4 tunneling,
ISATAP is designed for transporting IPv6 packets
within a site, not
ISATAP uses unicast addresses that include a 64-bit IPv6 prefix and a 64-bit interface identifier. The interface identifier
is created in modified EUI-64 format in which the first 32 bits contain the value 000:5EFE to indicate that the address is
an IPv6 ISATAP address. The table below describes an ISATAP address format.
Table 3. IPv6 ISATAP Address Format
link local or global IPv6 unicast prefix
IPv4 address of the ISATAP link
As shown in the table above, an ISATAP address consists of an IPv6 prefix and the ISATAP interface identifier. This interface
identifier includes the IPv4 address of the underlying IPv4 link. The following example shows what an actual ISATAP address
would look like if the prefix is 2001:DB8:1234:5678::/64 and the embedded IPv4 address is 10.173.129.8. In the ISATAP address,
the IPv4 address is expressed in hexadecimal as 0AAD:8108: