2.8.1. Address Format

IPv6 Address Format

Full:      2001:0DB8:0000:0001:0000:0000:0000:0001
Shortened: 2001:DB8:0:1::1

Rules:
1. Remove leading zeros in each group
2. Replace ONE run of consecutive all-zero groups with ::
   (the longest run; the first if two runs tie; never a lone 0 group)

IPv6 Configuration

Router(config)# ipv6 unicast-routing
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ipv6 address 2001:DB8:1:1::1/64
Router(config-if)# ipv6 address FE80::1 link-local
Router(config-if)# no shutdown

Verification:

Router# show ipv6 interface brief
GigabitEthernet0/0     [up/up]
    FE80::1
    2001:DB8:1:1::1

ipv6 unicast-routing is a global command and is off by default. Without it the router still addresses its interfaces and talks IPv6 on each link, but it will not forward IPv6 packets between them. ipv6 enable is a different command: it is an interface command that turns on IPv6 with only a link-local address, and it enables no routing.