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 www.iana.org/assignments/ipv6-unicast-address-assignments/ipv6-unicast-address-assignments.xml 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.
Organizations are increasingly transitioning to IPv6, the next generation protocol for defining how devices of all kinds communicate over networks. Now fully updated, IPv6 Fundamentals offers a thorough, friendly, and easy-to-understand introduction to the knowledge and skills you need to deploy and operate IPv6 networks.
Leading networking instructor Rick Graziani explains all the basics simply and clearly, step-by-step, providing all the details you'll need to succeed. You'll learn why IPv6 is necessary, how it was created, how it works, and how it has become the protocol of choice in environments ranging from cloud to mobile and IoT.
Graziani thoroughly introduces IPv6 addressing, configuration options, and routing protocols, including EIGRP for IPv6, and OSPFv3 (traditional configuration and with address families). Building on this coverage, he then includes more in-depth information involving these protocols and processes.
This edition contains a completely revamped discussion of deploying IPv6 in your network, including IPv6/IPv4 integration, dynamic address allocation, and understanding IPv6 from the perspective of the network and host. You'll also find improved coverage of key topics such as Stateless Address Autoconfiguration (SLAAC), DHCPv6, and the advantages of the solicited node multicast address.
Throughout, Graziani presents command syntax for Cisco IOS, Windows, Linux, and Mac OS, as well as many examples, diagrams, configuration tips, and updated links to white papers and official RFCs for even deeper understanding.
• Learn how IPv6 supports modern networks encompassing the cloud, mobile, IoT, and gaming devices
• Compare IPv6 with IPv4 to see what has changed and what hasn't
• Understand and represent IPv6 addresses for unicast, multicast, and anycast environments
• Master all facets of dynamic IPv6 address allocation with SLAAC, stateless DHCPv6, and stateful DHCPv6
• Understand all the features of deploying IPv6 addresses in the network including temporary addresses and the privacy extension
• Improve operations by leveraging major enhancements built into ICMPv6 and ICMPv6 Neighbor Discovery Protocol
• Configure IPv6 addressing and Access Control Lists using a common topology
• Implement routing of IPv6 packets via static routing, EIGRP for IPv6, and OSPFv3
• Walk step-by-step through deploying IPv6 in existing networks, and coexisting with or transitioning from IPv4
Reader Lucas Schultz says, "If you are looking to take the CCNA and IPv6 is not clear to you after Cisco material, this is the book you. This is the best in depth discuss about IPv6 how it works and the mechanisms to make it work. Author made it very easy to read and understand. Highly recommend this book to IT personnel or CCNA certification.
More Networking Protocols and Standards:
• Basic TCP/IP Networking
• PoE (Power Over Ethernet)
• Neighbor Discovery Protocol (NDP)
• IPv6 Prefix Length Notation
• Remote Control Protocols
• TCP/IP Utilities
• Internet Protocol versions IPv4, IPv5 and IPv6
• Video - Network Layer (Layer 3) of OSI Networking Model
• Free eBook: Introduction to 802.11 Wireless
• Video - The Upper Layers 5 Through 7 of the OSI Networking Model