IPv6 Network - 6over4 and ISATAP
The different automatic tunneling mechanisms all solve slightly different problems in slightly different ways. 6over4 and ISATAP mostly address tunneling IPv6 in IPv4 within a single organization or site network.1 6over4 does this by treating an IPv4 network as a fully functional IPv6 subnet, allowing regular address autoconfiguration. Unfortunately, 6over4 requires that the IPv4 infrastructure over which it runs support multicast. Because most IPv4 networks don’t support multicast routing, 6over4 hasn’t been deployed very widely, if at all. ISATAP on the other hand, foregoes multicast and treats the IPv4 network as a Non-Broadcast, Multiple Access (NBMA) network. Without multicasts, autoconfiguration and neighbor discovery don’t work, so ISATAP encodes the IPv4 address into the interface identifier part of the IPv6 address. This way, systems implementing ISATAP can easily determine which IPv4 host tunneled packets must be addressed to. The bottom 32 bits of an ISATAP interface identifier contain the IPv4 address, the top 32 bits are set to 02005EFE in hexadecimal when the IPv4 address is a regular, globally routable one and 00005EFE if the IPv4 address is a private address from one of the RFC 1918 ranges, thus preserving the meaning of the universal/local bit in the interface identifier. Figure 3-3 shows the relationship between the IPv4 and IPv6 addresses in ISATAP
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