Introduction

IPv6 Unicast is for communication between a single node and another single node – this is often called “one to one” communication. This type of address is only assigned to a single interface.

FYI: This lesson continues from the Introduction to IPv6 Address Types lesson.

There are five types of Unicast addresses. They are shown in order from their uses closest to the host to furthest away.

  • Special
  • Link-Local
  • Unique Local
  • Global Unicast
  • Compatibility

Special Unicast Address

There are two sub-types of Special Unicast Addresses - Loopback and Unspecified.

Loopback

The Loopback address is used to test the IP/TCP stack by sending a data packet to the interface of the corresponding loopback address and then it returns to the host. This packet never gets sent out of the interface. It is looped back to the host without ever leaving the device. It’s a test.
The 1st interface could have a Loopback address of ::1/128.
The 2nd interface could have a Loopback address of ::2/128.
And so on.

Unicast Loopback
The Loopback address is used to test the internal IP/TCP stack of a device.

Unspecified

The Unspecified address can be used:

  • to indicate a default route
  • as the source address of a node, while the node is in the process of acquiring its IP address

The Unspecified address is all zeroes (0:0:0:0:0:0:0:0:/128). It can be shortened to ::/128 or just ::.

Default Route

The default route is used when a packet’s destination address isn’t in the router’s routing table. The default route usually points towards the Internet.

Default Route

Unspecified Source Address

When a node is introduced to the network it may no addresses configured. If the address isn’t manually configured, the node needs to send a packet to other node(s) to request its address information. Since it doesn’t have an address yet, it will place the unspecified address of all zeroes into the source address field of the packet. This is done in the Link-Local Auto-Configuration and DHCPv6 processes.

Unicast Unspecified Source Address
Unspecified Source Address

All IPv6 enabled interfaces must have a link-local unicast address. LLAs are automatically assigned when an interface is enabled for IPv6. For a better design, LLAs can be manually assigned.

Link-Local Addresses (LLAs) are only used to communicate with nodes on the other side of a physical link.

Link-Local Unicast Address

The Link-Local address is used to send messages for:

  • neighbor and router discovery
  • routing protocols
  • discovering other network information from neighbors so that the device can route packets throughout the rest of the organization’s network and to the Internet

Pro Tip: It’s easy to spot a Link-Local Address. It will always start with FE80::/10 in the 1st quartet.

Unique-Local Unicast Address

Unique-Local Addresses (ULA) are routable within an organization, but they are not routable on the internet.

Unique-Local Unicast Address
Unique-Local Unicast Address

Pro Tip: Unique-Local Addresses will always begin with FC00::/7 in the 1st quartet.

Global Unicast Address

Global Unicast Addresses are routable on the internet.

Global Unicast Address (GUI)
Global Unicast Address (GUI)

The Global Unicast range is 2000 to 3FFF.

Pro Tip: Some people believe that Global Unicast Addresses compromise privacy and security. This is a myth! IPv6 offers increased privacy and security compared to legacy IPv4.

IPv4 Embedded Address

IPv4 Embedded Address​ type are an IPv4 to IPv6 transition mechanism.

There were two flavors of IPv4 Embedded Addresses, but now there is only one. IPv4-Compatible IPv6 Addresses are no longer used. Now only IPv4-mapped IPv6 addresses are supported.

An IPv4-mapped IPv6 Address can be used so that packets can travel from an IPv4 island to, or through, an IPv6 island.

It can also be used so a Dual Stack device can send a packet to an IPv4 only device.

The way it works is that the first 80 bits are all set to 0, the next 16 bits are set to 1 and the last 32 bits are the IPv4 address we're embedding/mapping into the IPv6 address.

With the IPv4 address of 10.10.10.10 the expanded IPv4 address would be:
0000:0000:0000:0000:0000:0000:ffff:10.10.10.10

This address shortened is:
::fff:10.10.10.10

Table of Unicast Address Ranges

Loopback ::1/128
Unspecified ::/128
Link-Local FE80::/10
Unique-Local FC00::/7
Global 2000 to 3FFF
IPv4-mapped IPv6 Address ::ffff:10.10.10.10 (last 32 bits are the IPv4 address)
Unicast Address Ranges

By looking at the address format, we can quickly know what kind of address we’re looking at - great for troubleshooting!

Cisco Press: IPv6 Address Representation and Address Types
RFC 4291 - IP Version 6 Addressing Architecture

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