The NPTv6 mapping translations that the firewall performs are checksum-neutral, meaning that “... they result in IP headers that will generate the same IPv6 pseudo-header checksum when the checksum is calculated using the standard Internet checksum algorithm [RFC 1071].” See RFC 6296, Section 2.6, for more information about checksum-neutral mapping.

If you are using NPTv6 to perform destination NAT, you can provide the internal IPv6 address and the external prefix/prefix length of the firewall interface in the syntax of the test nptv6 CLI command. The CLI responds with the checksum-neutral, public IPv6 address to use in your NPTv6 configuration to reach that destination.