Router. eth0 is external, with the 6to4 bound to it. eth1 is internal:
2: eth0: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qlen 1000
    inet6 fe80::200:24ff:fec8:dc50/64 scope link 
       valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qlen 1000
    inet6 2002:d976:2d51:2:0:24ff:fec8:dc51/48 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::200:24ff:fec8:dc51/64 scope link 
       valid_lft forever preferred_lft forever
7: 6to4@NONE: <NOARP,UP,10000> mtu 1480 
    inet6 2002:d976:2d51::1/16 scope global 
       valid_lft forever preferred_lft forever
    inet6 ::217.118.45.81/128 scope global 
       valid_lft forever preferred_lft forever


Internal host:
2: eth0: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qlen 1000
    inet6 2002:d976:2d51:2:20e:cff:fec3:20c2/64 scope global dynamic 
       valid_lft 2591756sec preferred_lft 604556sec
    inet6 fe80::20e:cff:fec3:20c2/64 scope link 
       valid_lft forever preferred_lft forever