The two types of source NAT translations that are supported in this scenario are translation of IPv4 hosts to IPv6 hosts with or without port address translation, and translation of one IPv6 subnet to another IPv6 subnet without port address translation. These are the types of source NAT translations that are supported by the Junos OS for IPv6 NAT. Translation of IPv4 hosts to IPv6 hosts allows IPv4-only hosts to communicate with IPv6-only hosts by changing the source IPv4 address to a corresponding IPv6 address. Port address translation can be optionally enabled to conserve IPv6 addresses by using different port numbers for different sessions. Translation of one IPv6 subnet to another IPv6 subnet allows IPv6 hosts to use a different IPv6 address range for outbound traffic, such as for security or policy reasons. Port address translation is not supported for this type of translation, as IPv6 addresses are abundant and do not need to be conserved. Reference: Juniper Security, Professional (JNCIP-SEC) Reference Materials source and documents: https://www.juniper.net/documentation/en_US/junos/topics/concept/security-nat-ipv6-overview.html