Hello,
The destination address :: is a IPv6 address, that is 0:0:0:0:0:0:0:0, this is an unspecified address, the same as the IPv4 address 0.0.0.0
If I understand you correctly, you want to translate the IPv4 address to IPv6 address, if yes, the 12.34.6.x/24 could be translated to the 6over4 IP address: FE80::0c22:06yy.
Note: FE80:: is determined by the link-local IPv6 address. For example, if your link-local IPv6 address is FE80:1111::1, then in this example, the first part should be FE80:1111::,
The second part 0c22:06yy is the hexadecimal of IPv4 address 12.34.6.x/24, the 12.34 is translated to 0c22, 6.x is translated to 06yy.