IPv6 Global Unicast Addresses by Rick Graziani

Global unicast addresses (GUAs) are globally routable and reachable in the IPv6 Internet; they are equivalent to public IPv4 addresses. GUA addresses are also known as aggregatable global unicast addresses. The GUA address is one of several types of IPv6 unicast addresses shown in Figure 5-1.

GUA addresses are equivalent to public IPv4 addresses.
The address range for GUA addresses is 2000::/3.
There are three parts to a GUA address:
   Global Routing Prefix
   Subnet ID
   Interface ID
A device can be configured with a global unicast address in these ways:
   Manual configuration
   Stateless Address Autoconfiguration (SLAAC)
   Stateful DHCPv6

This chapter takes a closer look at these and additional topics. We will begin by examining the structure of a global unicast address and how to manually configure a GUA address on Cisco IOS, Windows, Linux, and Mac OS.

This chapter shows a simple technique you can use to easily recognize the different parts of a GUA address for /48 prefixes, which I call the 3–1–4 rule. You will see that it is much easier to differentiate the prefix (network), subnet, and Interface ID (host portion) of an IPv6 address than in an IPv4 address.

We will see how the Subnet ID makes subnetting an IPv6 GUA address much simpler than subnetting IPv4. We will also see how to subnet beyond the Subnet ID by using bits from the Interface ID.

We will discuss the role of the Global Routing Prefix and the difference between provider-aggregatable (PA) and provider-independent (PI) address space. We will see how the IPv6 general prefix option can make readdressing on Cisco IOS easier.

The end of this chapter provides a brief overview of how a device can dynamically receive or create a GUA address using Stateless Address Autoconfiguration (SLAAC) or stateful DHCPv6. SLAAC and DHCPv6 are discussed more thoroughly in later chapters.

Structure of a Global Unicast Address

Figure 5-2 shows the generic structure of a global unicast address, which includes Global Routing Prefix, Subnet ID, and Interface ID.

The Internet Corporation for Assigned Names and Numbers (ICANN), the operator for the Internet Assigned Numbers Authority (IANA), allocates IPv6 address blocks to the five Regional Internet Registries (RIRs). The current global unicast address assignment from IANA begins with binary value 001, or the prefix 2000::/3. This results in a range of global unicast addresses of 2000::/3 through 3fff::/16.

How do you get this range of addresses using the 2000::/3 prefix? The /3 prefix length implies that only the first 3 bits are significant in matching the prefix 2000. The first 3 bits of the first hexadecimal value 2 are 001x. The fourth bit, x, is insignificant and can be either 0 or 1. This results in the first hextet being a 2 (0010) or a 3 (0011). The remaining 24 bits in the hextet (16-bit segment) can be 0 or 1. This is illustrated in Table 5-1

Currently, IANA limits global unicast address assignments to the range of 2000::/3, only one-eighth of the total IPv6 address space. IANA assignments from this block are registered in the IANA registry IPv6 Global Unicast Address Assignments; see If you look at the registry, you will notice that the RIRs also have prefixes shorter than /23, giving them even more addresses to allocate to Internet service providers (ISPs). We examine prefix lengths in more detail in a moment.

A global unicast address is configured on an interface, which can be configured with one or multiple GUA addresses. The GUA addresses can be on the same or different subnets, and they can be configured manually or obtained dynamically. It is not unusual to see multiple GUA addresses on a client OS device such as a Windows host.

So, which address is used when an interface has multiple GUA addresses and also a link-local address? This issue is addressed in RFC 3484, Default Address Selection for Internet Protocol Version 6 (IPv6).

It's important to remember that an IPv6 interface does not have to be configured with a global unicast address but it must have a link-local address. In other words, if an interface has a global unicast address, it also has a link-local address. However, if an interface has a link-local address, it does not necessarily have to have a global unicast address.

About the Author

Rick Graziani has been an instructor of computer networking and computer science courses at Cabrillo College in Aptos, California since 1994. Rick also teaches networking courses in the Computer Engineering department at the University of California, Santa Cruz and is on the Curriculum Engineering team for Cisco Networking Academy.

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