Global IPv6 Unicast Addresses Configuration

We have already discussed the Global IPv6 Address representation and structure. The Internet Assigned Numbers Authority (IANA) and The Internet Committee for Assigned Names and Numbers (ICANN) allocate Global IPv6 address blocks to the five Regional Internet Registries (RIRs). Only global unicast addresses with the first three bits of 001 or 2000::/3 are assigned to various Internet address registries. This is a tiny portion of available Global IPv6 addresses. A global IPv6 unicast address has three parts, which are illustrated in the figure below:-

• Global routing prefix
• Subnet ID
• Interface ID

This is the network portion of the global IPv6 address, which the provider has assigned. RIRs assign a /48 global routing prefix to customers, as shown in the figure. This can be used by everyone, from business networks to individual households. The figure illustrates the structure of a global unicast address using a /48 global routing prefix with a 16-bit subnet ID. The range of global IPv6 prefix in the first hextet is 0010 0000 0000 (2000) to 0011 1111 1111 (3FFF).

Static Configuration of Global IPv6 Address on Router

The Cisco routers’ configuration commands are similar for IPv4 and IPv6. The difference between both is the use of ipv6 in place of IP within the commands. The command used to configure an IPv6 global unicast address on any interface is “ipv6 address ipv6-address/prefix-length“ For example, the Configuration of IPv6 address on Cisco Router interface fa0/0 and fa0/1.

Static Configuration of Global IPv6 Address on Host

Similarly, we can configure an IPv6 address on a host computer like an IPv4. For example, As shown in Figure, the IP address configured for the host is 2001: DA1: B111:: ABCD: BCD: 1 and the default gateway address is 2001: DA1: B111:: ABCD: BCD: 1. Both addresses are global unicast addresses. The router’s link-local address can also be configured as the host’s default gateway. Both configurations will work. Static address configuration for the host is best practice in a small network, but for a more extensive network, dynamic assignment of IPv6 address configuration is best.

We can use dynamic IPv6 address configuration on host computers in two ways. The ways to configure IPv6 global unicast address automatically are Stateless Address Auto-Configuration (SLAAC) and Dynamic Host Configuration Protocol version 6 (DHCPv6). Using DHCPv6 or SLAAC, the local router’s link-local address will also automatically be specified as the default gateway address for the host.

Stateless Address Auto-Configuration (SLAAC)

It is a unique feature for IPv6 addresses that is not available in IPv4. Using SLAAC, the device can get an IPv6 address prefix, prefix length, default gateway address, and other information from an IPv6 router without using a DHCPv6 server. All Cisco devices have the capability of the SLAAC, but 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 send out ICMPv6 RA messages, after every 200 seconds, to all IPv6-enabled devices on the network. RA messages have three options to automatically get an IPv6 address. An RA message will also be sent in response to a host sending an Internet Control Messaging Protocol version 6 (ICMPv6) Router Solicitation (RS) message. IPv6 routing is not enabled by default. We can allow IPv6 routing using the following commands.