Cisco Cisco IOS Software Release 12.2(1c) Livre blanc

White Paper

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Figure 2. IPv6 Initiated Communication with Static Mapping

Step 1. A packet leaves Host A for its destination of Host B. The source address of the packet is

its actual IPv6 address (2001:9999:: 1). The destination IPv6 address is in the range of a

/96 prefix arbitrarily assigned on the NAT-PT router as the destination IPv6 address for

IPv6 to IPv4 communications. The prefix length is 96 bits long as it spares 32 bits for

addressing the entire IPv4 Internet. In our case, destination address is "PREFIX::1" as it

exists a static translation for this IPv6 address in the router.

Step 2. The packet reaches the NAT-PT router and the source and destination address of this

IPv6 incoming packet are translated following the static mapping available in the router.

An IPv4 packet is issued from the router with the following IPv4 addresses couple (source

172.17.1.1, destination 172.16.1.1).

Step 3. Packets originated from Host B to Host A swap IPv4 source and destination addresses.

Step 4. Using the same IPv6-IPv4 mapping as in Step 1, source and destination IPv4 addresses

are translated into IPv6 addresses.

IPv4 Initiated Communications

In the case of an IPv4 initiated communication, the Step 1 (above) is modified:

Step 1. A packet leaves Host A for its destination of Host B. The source address of the packet is

its actual IPv4 address (172.16.1.1). The destination IPv4 address is falling into an IPv4

subnet configured on the NAT-PT router. This IPv4 subnet configured on the router is

receiving the traffic targeted to the attached IPv6 cloud. In our case destination address is

172.17.1.1 and it exists a static translation to IPv6 for this IPv4 address in the router.

The rest of the process remains unchanged.

NAT-PT Communications with DNS-ALG

This paragraph describes a real-life communication through a NAT-PT router. It will clarify the

need for a DNS-ALG, as hosts are commonly accessed by name and not by IP address.

IPv6 Initiated Communications

IPv6 Host A wants to communicate with IPv4 Host B using its name instead of its IP address. The

first step of the communication is to resolve the name (www.a.com) of Host B. The DNS requests

flow is following the steps depicted in Figure 3.