IPv6 GUA Assignment

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.

shows the Internet Protocol Version 6 properties window and static IPv6 address and DNS server settings

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.

shows the Internet Protocol Version 6 properties window and automatic DHCP IPv6 address and DNS server settings

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 does not have 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 or the host has not been configured with at static IPv6 address.

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.

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. . . . . . . . . . . : 169.254.202.140
   Subnet Mask . . . . . . . . . . . : 255.255.0.0
   Default Gateway . . . . . . . . . : 
C:\PC1>

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.

an inverted tree diagram starts with Dynamic GUA Assignment, splits into two: stateless and stateful. Stateless splits into SLAAC and SLAAC with DHCPv6 server and stateful leades to a DHCPv6 stateful server

 

SLAAC