Describing IPv6overIPv4 Tunneling Mechanisms and IPv4 Addresses in IPv6 Format

This topic describes how IPv6-over-IPv4 encapsulation works and how to express IPv4 addresses in IPv6 format.

Cisco IOS Software Is IPv6-Ready: 6to4 Tunneling

6to4

• Is an automatic tunnel method

• Gives a prefix to the attached IPv6 network

The 6to4 tunneling method automatically establishes the connection of IPv6 islands through an IPv4 network. The 6to4 tunneling method applies a valid IPv6 prefix to each IPv6 island, which enables the fast deployment of IPv6 in a corporate network without address retrieval from the Internet service providers (ISPs) or registries.

The 6to4 tunneling method requires a special code on the edge routers, but the IPv6 hosts and routers inside the 6to4 site do not require new features to support 6to4. Each 6to4 site receives a /48 prefix, which is the concatenation of 0x2002 and the hexadecimal IPv4 address of the edge router.

For example, if the IPv4 address of the edge router is 192.168.99.1, the prefix of its IPv6 network is 2002:c0a8:6301::/48, because c0a86301 is the hexadecimal representation of 192.168.99.1. The IPv6 network can substitute any IP address in the space after the first 16-bit section (0x2002).

When an IPv6 packet with a destination address in the range of 2002::/16 reaches the 6to4 edge router, the 6to4 edge router extracts the IPv4 address that is embedded in the 2002:: destination address (inserted between the third and sixth octets, inclusive). The 6to4 router then encapsulates the IPv6 packet in an IPv4 packet with the destination IPv4 address that was extracted from inside the IPv6 destination address.

This IPv4 address represents the address of the other 6to4 edge router of the destination 6to4 site. The destination edge router decapsulates the IPv6 packet in the IPv4 packet and then forwards the native packet toward its final destination.

Note

2002::/16 is the address range specifically assigned to 6to4.

8-76

Building Scalable Cisco Internetworks (BSCI) v3.0

NAT-Protocol Translation (NAT-PT) is a translation mechanism that sits between an IPv6 network and an IPv4 network.

The job of the translator is to translate IPv6 packets into IPv4 packets and vice versa.

For legacy equipment that will not be upgraded to IPv6 and for some deployment scenarios, techniques that can connect IPv4-only nodes on IPv6-only nodes are available. Translation is basically an extension of NAT techniques.

NAT-PT

NAT-Protocol Translation (NAT-PT) is a translation mechanism that sits between an IPv6 network and an IPv4 network. The job of the translator is to translate IPv6 packets into IPv4 packets and vice versa.

The Stateless IP/Internet Control Message Protocol (ICMP) Translation (SIIT) algorithm translates the IP header fields. NAT handles the IP address translation. This example shows static NAT-PT translations. NAT-PT translations may also be mapped dynamically based on DNS queries using a DNS application level gateway (DNS ALG).

The example shows the translation of an IPv6 datagram sent from node A to node D. From the perspective of node A, it is establishing a communication to another IPv6 node. One advantage of NAT-PT is that no modifications are required on IPv6 node A; all it needs to know is the IPv6 address mapping to the IPv4 address of node D. This mapping can be obtained dynamically from the DNS server.

IPv4 node D can also send a datagram to node A by using the IPv4 address mapped to the IPv6 address of node A. Again, from the perspective of node D, it is establishing IPv4 communication with its correspondent. Again, node D requires no modification.

Other possible solutions are as follows:

■ ALGs: This method uses a dual-stack approach and enables a host in an IPv6-only domain to send data to another host in an IPv4-only domain. It requires that all application servers on a gateway run IPv6.

■ API: You can install a specific module in a host TCP/IP stack for every host on the network. The module intercepts IP traffic through an API and converts it for the IPv6 counterpart.

© 2006 Cisco Systems, Inc. Implementing IPv6 8-77

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