IPv6 address types, EUI-64, SLAAC and NDP

The address types

Global unicast addresses currently come from 2000::/3, which means they start with 2 or 3. Link-local addresses are created automatically on every IPv6 interface. They work only on that one link (the cable or Wi-Fi segment), and routers never forward them. Neighbour discovery and routing protocols such as OSPFv3 use them, and a router's link-local address is what hosts use as their default gateway.

Unique local addresses (ULA, RFC 4193) are the IPv6 version of private addresses. The defined range is fc00::/7, but only the fd00::/8 half is used, with a random 40-bit number after fd so that two companies that merge are unlikely to clash.

Multicast, anycast and no broadcast

IPv6 has no broadcast at all. Jobs that IPv4 did with broadcast are done with multicast, so only interested devices are disturbed. Well-known groups: ff02::1 is all nodes on the link, ff02::2 is all routers on the link. Each address also joins a solicited-node multicast group (ff02::1:ff followed by the last 24 bits of the address), which NDP uses instead of ARP broadcasts.

Anycast means the same unicast address is configured on several devices, and the network delivers a packet to the nearest one. Large DNS services use anycast so that users reach a server close to them. An anycast address looks exactly like a unicast address.

EUI-64: making an interface ID from a MAC address

A /64 network leaves 64 bits for the interface ID. A MAC address is only 48 bits, so modified EUI-64 stretches it: split the MAC in half, put fffe in the middle, and flip the 7th bit of the first byte (the universal/local bit, value 2).

1. MAC address 00:1A:2B:3C:4D:5E.
2. Split into halves: 001A2B and 3C4D5E.
3. Insert FFFE: 001A2B FFFE 3C4D5E.
4. First byte 00 is 00000000. Flip the 7th bit: 00000010, which is 02.
5. Interface ID: 021a:2bff:fe3c:4d5e. With the link-local prefix this gives fe80::21a:2bff:fe3c:4d5e.

Second example: MAC 0c:5a:9e:12:34:56. First byte 0c is 00001100; flipping the 7th bit gives 00001110, which is 0e. Interface ID 0e5a:9eff:fe12:3456.

How hosts get an address: SLAAC and DHCPv6

• SLAAC (Stateless Address Autoconfiguration): the router sends a Router Advertisement with the /64 prefix. The host adds its own interface ID (EUI-64 or random) and checks nobody else uses it. No server keeps track, which is why it is called stateless.
• Stateless DHCPv6: the host makes its address with SLAAC, then asks a DHCPv6 server only for extra information such as DNS servers. The RA has the O (other configuration) flag set.
• Stateful DHCPv6: a DHCPv6 server hands out addresses and remembers who has which, like DHCP in IPv4. The RA has the M (managed) flag set.

NDP: Neighbor Discovery Protocol

NDP uses ICMPv6 messages and replaces ARP and more. There are four messages to know, plus Redirect.

Cisco IOS configuration

R1(config)# ipv6 unicast-routing
R1(config)# interface GigabitEthernet0/0
R1(config-if)# ipv6 address 2001:db8:1:1::1/64
R1(config-if)# ipv6 address fe80::1 link-local
R1(config-if)# no shutdown
R1(config-if)# interface GigabitEthernet0/1
R1(config-if)# ipv6 address 2001:db8:1:2::/64 eui-64
R1(config-if)# no shutdown
R1(config-if)# interface GigabitEthernet0/2
R1(config-if)# ipv6 address autoconfig
R1(config-if)# end
R1# show ipv6 interface brief
GigabitEthernet0/0     [up/up]
    FE80::1
    2001:DB8:1:1::1
GigabitEthernet0/1     [up/up]
    FE80::21A:2BFF:FE3C:4D5E
    2001:DB8:1:2:21A:2BFF:FE3C:4D5E

Common mistakes

• Forgetting to flip the 7th bit in EUI-64: 00:1A:2B... gives 021a, not 001a.
• Expecting to ping a link-local address without saying which interface: the same fe80:: prefix exists on every link, so the device must be told which one to use.
• Looking for an IPv6 broadcast address. There is none; use multicast groups such as ff02::1.