IPv6 Global Unicast Address Assignment
IPv6 Global Unicast Address Assignment

IPv6 Global Unicast Address Assignment


This topic explain how an IPv6 host can acquire its IPv6 configuration. Start learning CCNA 200-301 for free right now!!

Note: Welcome: This topic is part of Module 8 of the Cisco CCNA 2 course, for a better follow up of the course you can go to the CCNA 2 section to guide you through an order.

IPv6 Host Configuration

First things first. To use either stateless address autoconfiguration (SLAAC) or DHCPv6, you should review global unicast addresses (GUAs) and link-local addresses (LLAs). This topic covers both.

On a router, an IPv6 global unicast address (GUA) is manually configured using the ipv6 address ipv6-address/prefix-length interface configuration command.

A Windows host can also be manually configured with an IPv6 GUA address configuration, as shown in the figure.

IPv6 Host Configuration
IPv6 Host Configuration

Manually entering an IPv6 GUA can be time consuming and somewhat error prone. Therefore, most Windows host are enabled to dynamically acquire an IPv6 GUA configuration, as shown in the figure.

IPv6 GUA configuration
IPv6 GUA configuration

IPv6 Host Link-Local Address

When automatic IPv6 addressing is selected, the host will attempt to automatically obtain and configure IPv6 address information on the interface. The host will use one of three methods defined by the Internet Control Message Protocol version 6 (ICMPv6) Router Advertisement (RA) message received on the interface. An IPv6 router that is on the same link as the host sends out RA messages that suggest to the hosts how to obtain their IPv6 addressing information. The IPv6 link-local address is automatically created by the host when it boots and the Ethernet interface is active. The example ipconfig output shows an automatically generated link-local address (LLA) on an interface.

In the figure, notice that the interface did not create an IPv6 GUA. The reason is because, in this example, the network segment does not have a router to provide network configuration instructions for the host.

Note: Host operating systems will at times show a link-local address appended with a “%” and a number. This is known as a Zone ID or Scope ID. It is used by the OS to associate the LLA with a specific interface.

Note: DHCPv6 is defined in RFC 3315.

C:\PC1> ipconfig
Windows IP Configuration
Ethernet adapter Ethernet0:
   Connection-specific DNS Suffix  . : 
   IPv6 Address. . . . . . . . . . . : 
   Link-local IPv6 Address . . . . . : fe80::fb:1d54:839f:f595%21
   IPv4 Address. . . . . . . . . . . :
   Subnet Mask . . . . . . . . . . . :
   Default Gateway . . . . . . . . . : 

IPv6 GUA Assignment

IPv6 was designed to simplify how a host can acquire its IPv6 configuration. By default, an IPv6-enabled router advertises its IPv6 information. This allows a host to dynamically create or acquire its IPv6 configuration.

The IPv6 GUA can be assigned dynamically using stateless and stateful services, as shown in the figure.

All stateless and stateful methods in this module use ICMPv6 RA messages to suggest to the host how to create or acquire its IPv6 configuration. Although host operating systems follow the suggestion of the RA, the actual decision is ultimately up to the host.

IPv6 GUA Assignment
IPv6 GUA Assignment

Three RA Message Flags

The decision of how a client will obtain an IPv6 GUA depends on the settings within the RA message.

An ICMPv6 RA message includes three flags to identify the dynamic options available to a host, as follows:

  • A flag – This is the Address Autoconfiguration flag. Use Stateless Address Autoconfiguration (SLAAC) to create an IPv6 GUA.
  • O flag – This is the Other Configuration flag. Other information is available from a stateless DHCPv6 server.
  • M flag – This is the Managed Address Configuration flag. Use a stateful DHCPv6 server to obtain an IPv6 GUA.

Using different combinations of the A, O and M flags, RA messages inform the host about the dynamic options available.

The figure illustrates these three methods.

Three RA Message Flags

Glossary: If you have doubts about any special term, you can consult this computer network dictionary.

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