Introduction to IPv6 · Topic 3 of 3

IPv6 Address Types and Special Addresses

Learn the IPv6 address types, including global unicast, link local, unique local, multicast and anycast, and the special and reserved addresses.

4 min

In a nutshell

IPv6 has unicast, multicast and anycast addresses, and no broadcast.
Global unicast addresses come from 2000::/3.
Link local addresses are fe80::/10 and are never routed.

What it is

IPv6 sorts addresses into unicast, multicast and anycast, and each type has recognisable prefixes.

A unicast address names one interface. A multicast address names a group of interfaces. An anycast address is shared by several devices, and a packet goes to the nearest one.

There is no broadcast type. Multicast does that job instead.

How it works

You can tell the type of an address from the first bits of the prefix.

Unicast address scopes

Type	Prefix	Use
Global unicast	`2000::/3`	Routable on the Internet
Link local	`fe80::/10`	One link only, never routed
Unique local	`fc00::/7`	Private use inside a site, in practice `fd00::/8`

Global unicast addresses are the public ones. Today they come from the 2000::/3 block, so they start with a 2 or a 3.

Link local addresses exist on every IPv6 interface, and a router never forwards them. They are used for Neighbor Discovery and for next hop addresses in routing. RouterOS creates one on each interface automatically.

Unique local addresses play the role of private IPv4 ranges. They are not announced to the Internet. A site picks a random 40 bit value after fd to keep its ranges unlikely to collide with another site.

On RouterOS you can see the type in the flags of the address list.

/ipv6 address print

A link local entry carries the L flag, and a global entry carries the G flag.

Multicast addresses

Multicast addresses start with ff00::/8. The last hex digit of the first group, such as the 2 in ff02, holds the scope, for example link local or site.

Address	Meaning
`ff02::1`	All nodes on the link
`ff02::2`	All routers on the link
`ff02::1:ff00:0/104`	Solicited node range
`ff02::5`	All OSPF routers
`ff02::9`	All RIP routers

The solicited node address is built from the last 24 bits of a unicast address. Neighbor Discovery uses it to find a neighbour without bothering every host.

Anycast addresses

An anycast address looks exactly like a unicast address. Several devices are configured with the same address, and routing delivers packets to the closest one. Public DNS services often use this idea.

Special and reserved addresses

Address	Meaning
`::/128`	Unspecified address, used as a source before a host has an address
`::1/128`	Loopback, the host itself
`::ffff:0:0/96`	IPv4 mapped addresses
`2001:db8::/32`	Reserved for documentation and examples
`2002::/16`	Used by the 6to4 tunnelling mechanism
`::/0`	Default route, not an address

The unspecified address appears as the source of a host that is checking for duplicates. It is never a destination.

The documentation range is the reason this guide uses 2001:db8 in all examples. You should never see it routed on the Internet.

A host that starts up with no address and sends a Neighbor Solicitation uses :: as its source. That is how the duplicate check avoids claiming an address it does not yet own.

/ipv6 route add dst-address=::/0 gateway=2001:db8:1::1

The default route is written ::/0, the equal of 0.0.0.0/0 in IPv4.

Exam tip

To decide the type of an address quickly, read the first digits. fe80 is link local, fd is unique local, ff is multicast, and 2 or 3 is global unicast.

What to remember for the exam

Eight facts cover most questions on this topic.

IPv6 has unicast, multicast and anycast addresses, and no broadcast.
Global unicast addresses come from 2000::/3.
Link local addresses are fe80::/10 and are never routed.
Unique local addresses are fc00::/7, and in practice start with fd.
Multicast addresses start with ff, for example ff02::1 for all nodes.
::1 is loopback and :: is the unspecified address.
2001:db8::/32 is reserved for documentation.
The IPv6 default route is ::/0.

Watch out

Link local addresses repeat on every interface, so a next hop of fe80::1 is meaningless without naming the interface.

Common mistakes

These four mistakes appear often.

Routing a link local address. Routers never forward fe80::/10, so it cannot reach another network.

Confusing unique local with link local. Unique local addresses can be routed inside a site. Link local addresses stay on one link.

Treating anycast as a special format. It is only a unicast address configured on several devices.

Using the documentation prefix in production. The 2001:db8::/32 range is for examples and will not be routed on the Internet.

Check yourself

Work through these three questions.

Which prefix do link local addresses use, and can they be routed?
They use fe80::/10, and routers never forward them off the link.
What is the IPv6 equivalent of the IPv4 address 0.0.0.0/0 default route?
The default route is written ::/0.
What is the difference between ::1 and ::?
::1 is the loopback address of the host, and :: is the unspecified address used as a source before the host has an address.

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