IPv6 address notation
A full IPv6 address has eight groups of four hexadecimal digits, for example 2001:0db8:0000:0000:0000:ff00:0042:8329. Leading zeros in each group can be dropped, and one run of consecutive all-zero groups can be replaced with ::, giving 2001:db8::ff00:42:8329.
RFC 5952 defines the canonical compressed form: lowercase letters, the longest zero run compressed, and :: never used for a single group. Use the expanded form when comparing strings or building reverse DNS (ip6.arpa) names.
Preguntas frecuentes
Can :: appear twice?
No. It would be ambiguous how many zero groups each :: stands for.
What is a link-local address?
Addresses in fe80::/10 are valid only on the local network segment and are created automatically on every IPv6 interface.