 (config-if)#ipv6 address AAAA AAAA:AAAA 1::/64 eui-64 This is fine in a lab environment, but remember that in the real world, you should have a properly allocated /64 prefix given to you by your provider, ARIN, or some other authority. Now the interface will have a network address of aaaa : aaaa : aaaa :1:: /64, and the host address will be the EUI-64 address resolved from the interface's MAC address. The :1: represents the subnet I've allocated from the /48 range aaaa:aaaa:aaaa. We can see this with the show ipv6 interface interface-name command: R1#sho ipv6 int fo/o FastEtherneto/o is up, line protocol is up IPv6 is enabled, link-local address is sFE80::212:43FF:FEB9:2C70 Global unicast address( (es): AAAA:AAAA:AAAA:1:212:43FF:FEB9:2C70,subnet is AAAA:AAAA:AAAA:1::/64 [EUI] Joined group address (es): FF02 1 FF02 :2 FF02 : :1: FF00: FF02 : :1: FFB9 : 2C70 MTU is 1500 bytes ICMP error messages limited to one every 100 milliseconds ICMP redirects are enabled ND DAD is enabled, number of DAD attempts: 1 ND reachable time is 30000 milliseconds ND advertised reachable time is 0 milliseconds ND advertised retransmit interval is 0 milliseconds ND router advertisements are sent every 200 seconds ND router advertisements live for 1800 seconds Hosts use stateless autoconfig for addresses. Simple Router Configuration The interface now has a link-local address of FE80::212:43FF FEB9 2C70, SO defined because of the FE80::2 prefix, and a global unicast address of AAAA :AAAA:AAAA: 1:212:43FF FEB9 2C70. The last 14 hex digits are the same on both addresses, because they both reflect the MAC address of the interface (with the added FF:FE in the middle). Let's think about this for a minute. Imagine if all devices on the network had been configured this way. If we wanted to renumber, all we would need to change would be the network address. We no longer need to allocate IP addresses, no one needs to maintain a continuously out-of-date spreadsheet, and no one will have any idea what the address of any host is. Wait, that last one 