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3-15
Cisco ASA Series Firewall ASDM Configuration Guide
Chapter 3 Information About NAT (ASA 8.3 and Later)
NAT and IPv6
NAT and IPv6
You can use NAT to translate between IPv6 networks, and also to translate between IPv4 and IPv6
networks (routed mode only). We recommend the following best practices:
• NAT66 (IPv6-to-IPv6)—We recommend using static NAT. Although you can use dynamic NAT or
PAT, IPv6 addresses are in such large supply, you do not have to use dynamic NAT. If you do not
want to allow returning traffic, you can make the static NAT rule unidirectional (twice NAT only).
• NAT46 (IPv4-to-IPv6)—We recommend using static NAT. Because the IPv6 address space is so
much larger than the IPv4 address space, you can easily accommodate a static translation. If you do
not want to allow returning traffic, you can make the static NAT rule unidirectional (twice NAT
only). When translating to an IPv6 subnet (/96 or lower), the resulting mapped address is by default
an IPv4-embedded IPv6 address, where the 32-bits of the IPv4 address is embedded after the IPv6
prefix. For example, if the IPv6 prefix is a /96 prefix, then the IPv4 address is appended in the last
32-bits of the address. For example, if you map 192.168.1.0/24 to 201b::0/96, then 192.168.1.4 will
be mapped to 201b::0.192.168.1.4 (shown with mixed notation). If the prefix is smaller, such as /64,
then the IPv4 address is appended after the prefix, and a suffix of 0s is appended after the IPv4
address. You can also optionally translate the addresses net-tonet, where the first IPv4 address maps
to the first IPv6 address, the second to the second, and so on.
• NAT64 (IPv6-to-IPv4)—You may not have enough IPv4 addresses to accommodate the number of
IPv6 addresses. We recommend using a dynamic PAT pool to provide a large number of IPv4
translations.
For specific implementation guidelines and limitations, see the configuration chapters.
How NAT is Implemented
The ASA can implement address translation in two ways: network object NAT and twice NAT. This
section includes the following topics:
• Main Differences Between Network Object NAT and Twice NAT, page 3-15
• Information About Network Object NAT, page 3-16
• Information About Twice NAT, page 3-16
Main Differences Between Network Object NAT and Twice NAT
The main differences between these two NAT types are:
• How you define the real address.
–
Network object NAT—You define NAT as a parameter for a network object. A network object
names an IP host, range, or subnet so you can then use the object in configuration instead of the
actual IP addresses. The network object IP address serves as the real address. This method lets
you easily add NAT to network objects that might already be used in other parts of your
configuration.
–
Twice NAT—You identify a network object or network object group for both the real and
mapped addresses. In this case, NAT is not a parameter of the network object; the network object
or group is a parameter of the NAT configuration. The ability to use a network object group for
the real address means that twice NAT is more scalable.