Configuration of Global IPv6 Unicast Address

Static Configuration – Router

Most of the configuration commands in the Cisco routers are similar for both IPv4 and IPv6. The only difference is the use of ipv6 in place of IP within the commands. The command to configure an IPv6 global unicast address on any interface is “ipv6 address ipv6-address/prefix-length “

Example Configuration on Router fa0/0 and fa0/1 interfaces


Router#configure terminal

Router(config)interface fa/0/0

Router(config-if)#ipv6 address 2001:DB8A:AACA:A::1/64

Router(config-if)no shutdown


Router(config)interface fa/0/1

Router(config-if)#ipv6 address 2001:DB8A:AACA:B::1/64

Router(config-if)no shutdown


Router(config) do wr

Building configuration…


Static Configuration- Host

Manual configuration of the IPv6 address is similar to configure an IPv4 address. As shown in Figure, the default gateway address configured for PC1 is 2001:DB8A:AACA:A::1. This is the global unicast address of the Router Fast Ethernet interface 0/0 on the same network. Link-local address of the router can also be configured as the gateway for the host. Both configurations will work.


Configuring static addresses on clients is best for a small network. For larger network dynamic assignment of IPv6 addresses is best practice.

There are two ways in which a device can get an IPv6 global unicast address automatically:

  • Stateless Address Auto Configuration (SLAAC)
  • Dynamic Host Configuration Protocol version 6 (DHCPv6)

Using DHCPv6 or SLAAC, the local router’s link-local address will automatically be specified as the default gateway address for the host.

Dynamic Configuration – SLAAC

The SLAAC is a unique feature for IPv6 addresses. Stateless address configuration means that a device to obtain its prefix, prefix length, default gateway address, and other information from an IPv6 router without the use of a DHCPv6 server. All Cisco devices have the capability of the SLAAC. By default, SLAAC does not provide anything to the client outside of an IPv6 address and a default gateway. Using SLAAC, devices rely on the local router’s ICMPv6 Router Advertisement (RA) messages to obtain the necessary information.

IPv6 enabled routers to send out ICMPv6 RA messages after every 200 seconds, to all IPv6-enabled devices on the network. An RA message will also be sent in response to a host sending an ICMPv6 Router Solicitation (RS) message.

IPv6 routing is not enabled by default. To enable a router IPv6 following command will be used.


Router#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#ipv6 unicast-routing


The ICMPv6 RA Message

The ICMPv6 RA message is a suggestion to a device on how to get an IPv6 global unicast address. The device operating system is final authority to get the IPv6 address. The ICMPv6 RA message consists of:

  • Network prefix and prefix length
  • Default gateway address
  • DNS addresses and domain name

There are three options for RA messages which used to get an IPv6 address automatically:


2. SLAAC with a stateless DHCPv6 server

3. DHCPv6 (no SLAAC)

All Three option will be discussed in next lesson.

Structure of Global IPv6 Addresses

Currently, Internet Assigned Numbers Authority (IANA) and The Internet Committee for Assigned Names and Numbers (ICANN) allocates IPv6 address blocks to the five RIRs. Only global unicast addresses with the first three bits of 001 or 2000::/3 are being assigned to various Internet address registries. This is only very small portion of available IPv6 addresses. A global unicast address has three parts which are illustrated in the figure below:-

  • Global routing prefix
  • Subnet ID
  • Interface ID

Global Routing Prefix

This is the network portion of the global IPv6 address, which is assigned by the provider, such as an ISP, to a customer or site. In general, RIRs assign a /48 global routing prefix to customers as shown in the above figure. This can use everyone from business networks to individual households.

global routing prefix

The figure shows the structure of a global unicast address using a /48 global routing prefix with 16-bit subnet ID. /48 prefixes are the most common global routing prefixes assigned. The range of global IPv6 prefix in the first hextet is 0010 0000 0000 (2000) to 0011 1111 1111 (3FFF).

Subnet ID

The Subnet ID is used by departments to recognize subnets within its site. The larger the subnet ID, the more subnets available.

Interface ID

This is the host portion of the IPv6 address like the host portion of an IPv4 address. The term Interface ID is used for the reason that a single host may have multiple interfaces, each having one or more IPv6 addresses. It is extremely recommended that in most cases /64 subnets should be used.

Example of IPv6 Address


shown in the above IP address, in a /64 global unicast address the first four hextets are for the network portion of the address which is indicated in red, with the fourth hextets indicating the Subnet ID. The remaining four hextets are for the Interface ID.