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
command. The CLI responds with the checksum-neutral, public IPv6
address to use in your NPTv6 configuration to reach that destination.