IPv6 NPTv6 (Network Prefix Translation)

This topic is to discuss the following lesson:

Why did I use a loopback with a prefix instead of prefix 2001:DB8:0:23::/64 (the link between NPTv6 and H3)? I tried this the first time but it doesn’t work because H3 will do a neighbor solicitation for 2001:DB8:0:23::1/64 (the translated address). Since nobody responds to that address, the ping fails.

I guess the question is - what if you changed the IP of H1 to 2001:DB8:0:12::2/64 (and swapped G2 to ::1)

Then the translated address for H1 would be 2001:DB8:0:23::2/64 - so NPTV6 should respond to the neighbour solicitation :wink:

Hello Chris.

Yes that makes sense. You might want to try to lab it for confirmation and let us know of your results…


I don’t understand the use of this prefix
If the prefix is give by a ISP it do not belong to the Customer and if you are the owner of the prefix you can use BGP to still make it available
Is there a pratical scenario where the is an advantage of using NPTv6


Hello Fabrice

IPv4 and IPv6 are similar in that they separate their respective addresses into two sections. The terminology used is somewhat different however. Where an IPv4 address is separated into the network portion and the host portion using a subnet mask, an IPv6 address is separated into a prefix and a host identifier using the prefix length.

So for an IP address of 2001:DB8:0:12::1/64 as in the lesson, the prefix is 2001:DB8:0:12, the host identifier is ::1 and the prefix length is 64. 64 indicates what part of the address is the prefix, that is, the first 64 bits.

So, when we say that we are applying network prefix translation, what we’re doing is taking the 2001:DB8:0:12 part of the address and replacing it with another prefix, and in this lesson, this other prefix is 2001:DB8:0:23.

So for every packet that comes in to a destination IP address of 2001:DB8:0:12::1, the NPTv6 router will replace the 2001:DB8:0:12 with 2001:DB8:0:23. The resulting mapping will be as follows:

2001:DB8:0:12::1 will map to 2001:DB8:0:23::1
2001:DB8:0:12::2 will map to 2001:DB8:0:23::2
2001:DB8:0:12::3 will map to 2001:DB8:0:23::3
2001:DB8:0:12::4 will map to 2001:DB8:0:23::4
2001:DB8:0:12::5 will map to 2001:DB8:0:23::5

So this is indeed a one to one mapping.

Now the prefix may be owned by you, or it may be the prefix that the ISP is giving you as part of your connectivity package. The NPT need not be applied at the edge of your network, it may even be applied inside your network.

Some practical scenarios where this can be useful are included in the lesson. Specifically:

  • Address independence: you don’t have to change your IPv6 prefixes on your local network when your global IPv6 prefix changes. On the other hand, IPv6 renumbering is not so bad compared to IPv4.
  • ULAs (Unique Local Addresses): NPTv6 translates the prefix in your ULAs to a global prefix that is routable on the Internet.
  • Access-lists: Your host has two IPv6 addresses and only one of them is permitted through some firewall. Your host won’t know which source address is permitted through the firewall so by using NPTv6, you can translate the address to a prefix that is permitted through the firewall.

The IETF stipulates that NPTv6 provides what they call “address independence.” More about this can be found here:

I hope this has been helpful!


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I don’t understand discussions of question “What are limitations in use of NPTv6 for IPv6 vs IPv6 address translation?”. Some people suppose that one of limitation is “1-to-1 prefix rewrite”. But other suppose that limitation is “mismatched prefix allocations”.

It seems both versions are true. Anybody clarify me?


Hello Boris

Limitations to any technology can be a subjective matter. For one person, one aspect may be limiting while another may not. Compared to NAT, the fact that NPTv6 can only provide a one-to-one translation may be a limitation for someone that wants this functionality, however, NPTv6 was not developed for this purpose. For others, the fact that you can’t have mismatched prefix allocation sizes is a limit.

NPTv6 was developed in order to provide a very simple functionality: rewrite the IPv6 prefix. That’s it. Now for some this is a limitation, for others, this is exactly what they want it to do.

Some things that are limitations, regardless of how it functions or what it’s purpose is, is the fact that IPsec cannot be used across an NPTv6 translation, and a more complex DNS setup is required, specifically, split-horizon DNS must be applied.

I hope this has been helpful!


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Hello Laz,

Thanks a lot. The explanation has been helpful!

Hey Laz and Rene,

Why there isn’t any show command like show nat66 translations?

I know that the conversion is 1:1 but sometimes you want to see which ip address was translated and which wasn’t , and you can’t verify this with the other show commands.
(For example - port forwarding in NPTv6 for servers environment with public services)

Another question that I wanna ask: if there is any method like NAT ALG in the NPTv6 version?

Thanks you very much

Hello Nitay

As far as I can see there is no way to see the actual translations of the specific IP addresses. But there is a reason for this. NAT66 does not keep track of each individual translation like NAT for IPv4 does. It simply translates the prefixes. In this sense, it is stateless as far as the specific addresses are concerned, that’s why you can’t see a record of the translations. The only way to see and verify specific translations is to do a wireshark capture and see what addresses appear in the IPv6 header.

ALG does exist for NAT66 in much the same way as it does for IPv4 NAT. This is emphasized in RFC 6296 describing IPv6 to IPv6 Network Prefix Translation where it states that:

NPTv6 may interfere with the use of application
protocols that transmit IP addresses in the application-specific
portion of the IP datagram. These applications currently require
Application Layer Gateways (ALGs) to work correctly through NAPT44
devices, and similar ALGs may be required for these applications to
work through NPTv6 Translators.

However, from my research, I find that most Cisco devices that support NAT66 don’t simultaneously support ALG. For example, this Cisco documentation on NPTv6 states that:

Application Level Gateways (ALG) is not supported by NPTv6 support on ASR1k/CSR1k/ISR4k feature. Payload address or port translation is not supported.

I have been unable to find documentation indicating a Cisco device supporting ALG on NAT66 translations.

I hope this has been helpful!


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