Hello Inon
If we have an IPv4 address of 192.168.23.3 as in the lesson, then the automatic 6to4 tunnelling will use 2002:C0A8:1703 as the beginning of the IPv6 address. Now from this, we can use any subnet prefix of /48 to /64 to represent the IPv6 addresses behind each end of the tunnel. So in a sense, 2002:C0A8:1703::/48 represents the tunnel interface on R1. All IPv6 addresses behind R1 will be mapped to the appropriate address. For example:
We ping from 2001::3/128 to 2001::1/128. When the ping reaches R3, it will be converted to 2002:C0A8:1703::1, sent over the tunnel to R1 where it will be converted back to 2001::1.
When you are referring to specific hosts, you would use the host mask of /128, but when referring to the tunnelling prefix, you will always use the /48 prefix.
Now the general-prefix command automatically configures the prefix as /48 but you are able to use a /64 or anything in between if you like.
I hope this has been helpful!
Laz