hi guys, i m preparing for my ccnp bsci but i m not at all able to understand ipv6, i mean that like in ipv4 we have certain rules regarding subnet mask or prefix length and we could easily write our own addresses for practice, but i m clearly not able to understand tht wat rules are for ipv6 prefixes ? wat shld be the min length or max length ?? or if i m writing my own ipv6 address wat prefix length i can take ??? plz some1 help me with a document on cisco or anyother site that refers to my problem i will really be gratefull,

thanks in advance