IPv6 Address Types

What it is

IPv6 has several kinds of address, and the first digits of an address tell you what it is for.

Learning to recognise them tells you at once how far an address can travel.

The address types in this objective are global unicast, unique local, link-local, anycast and multicast. It also covers modified EUI-64, a method for building the host part of an address from a MAC address.

How it works

Each address type has its own prefix, scope and purpose.

Unicast addresses

A unicast address identifies a single interface. A packet sent to it goes to that one interface. Three kinds matter here.

Global unicast addresses are the public addresses of IPv6. They start with 2 or 3 in practice, since the block is 2000::/3. They are unique across the internet and can be routed globally.

Unique local addresses are the IPv6 counterpart to private IPv4 addresses. They are meant for use inside a site or between sites, and are not routed on the public internet. In practice, you see addresses beginning with FD.

Link-local addresses start with FE80. Every IPv6 interface has one. It is valid only on the directly connected link, and routers do not forward packets from it.

Link-local addresses are used for neighbour discovery, and as the next-hop address in routing protocols. Because the same FE80 prefix exists on every link, a router interface needs to be named to tell them apart.

Two other unicast addresses are worth knowing: the loopback address ::1, which a device uses to talk to itself, and the unspecified address ::, used when a device has no address yet.

Multicast

A multicast address identifies a group of interfaces. A packet sent to it is delivered to every member of the group.

All multicast addresses start with FF00::/8. Some well-known ones:

• FF02::1 reaches all nodes on the link.
• FF02::2 reaches all routers on the link.
• Solicited-node multicast, from FF02::1:FF00:0/104, is built from the last 24 bits of a unicast address. It is how a device asks for another device's link-layer address, replacing the ARP broadcast.

IPv6 has no broadcast. Multicast does the same job more efficiently, because only devices that belong to the group process the packet.

Anycast

An anycast address is assigned to more than one device, usually in different places. A packet sent to the anycast address goes to the nearest device, as measured by the routing protocol.

It is used for services such as DNS to answer from the closest server. Anycast addresses come from the unicast range, so an address is only anycast because of how it is configured.

On a Cisco router, this is done with the anycast keyword:

interface GigabitEthernet0/0
 ipv6 address 2001:db8:1:1::100/64 anycast

Modified EUI-64

EUI-64 builds a 64-bit interface ID from a 48-bit MAC address. It follows three steps:

1. Split the MAC address in the middle.
2. Insert FFFE between the two halves.
3. Flip the seventh bit of the first byte. That is the universal/local bit.

An example, using the MAC address 00:50:56:AA:BB:CC:

• Split and insert FFFE: 0050:56FF:FEAA:BBCC
• The first byte is 00, which is 00000000 in binary. Flipping the seventh bit gives 00000010, which is 02.
• Result: 0250:56FF:FEAA:BBCC

With a /64 prefix of 2001:db8:1:1::/64, the full address is 2001:db8:1:1:250:56ff:feaa:bbcc.

On a Cisco router you apply it with ipv6 address 2001:db8:1:1::/64 eui-64. Link-local addresses on many devices are built the same way.