server:

ifconfig gif1 create
ifconfig gif1 tunnel 10.31.15.225 10.31.1.2
ifconfig gif1 inet6 alias 2a01:48:100:13b:ffff:ffff:ffff:0 2a01:48:100:13b:ffff:ffff:ffff:1 prefixlen 128
route add -inet6 2a01:48:100:13b:ffff:ffff:ffff:1 -interface gif1
route add -inet6 2a01:48:100:13b:1::/80 -interface gif1

client:

ifconfig gif0 create
ifconfig gif0 tunnel 10.31.1.2 10.31.15.225
ifconfig gif0 inet6 alias 2a01:48:100:13b:ffff:ffff:ffff:1 2a01:48:100:13b:ffff:ffff:ffff:0 prefixlen 128
route add -inet6 :: -interface gif0
route add -inet6 2a01:48:100:13b:ffff:ffff:ffff:0 -interface gif0
route add -inet6 2a01:48:100:13b:1:: -interface fxp0