Chapter 17: IP Version 6

Cisco Press

Global Unicast Addressing, Routing, and Subnetting: This section introduces the concepts behind unicast IPv6 addresses, IPv6 routing, and subnetting using IPv6, all in comparison to IPv4.

IPv6 Protocols and Addressing: This section examines the most common protocols used in conjunction with IPv6.

Configuring IPv6 Routing and Routing Protocols: This section shows how to configure IPv6 routing and routing protocols on Cisco routers.

IPv6 Transition Options: This section explains some of the options for migrating from IPv4 to IPv6.

IP version 6 (IPv6), the replacement protocol for IPv4, is well known for a couple of reasons. IPv6 provides the ultimate solution for the problem of running out of IPv4 addresses in the global Internet by using a 128-bit address—approximately 1038 total addresses, versus the mere (approximate) 4*109 total addresses in IPv4. However, IPv6 has been the ultimate long-term solution for over ten years, in part because the interim solutions, including Network Address Translation/Port Address Translation (NAT/PAT), have thankfully delayed the day in which we truly run out of public unicast IP addresses.

This chapter focuses on IPv6 addressing and routing, in part because the primary motivation for the eventual migration to IPv6 is to relieve the address constraints of IPv4. This chapter also briefly introduces some of the other features of IPv6, as well as explains some of the reasons for the need for IPv6.

"Do I Know This Already?" Quiz

The "Do I Know This Already?" quiz allows you to assess whether you should read the entire chapter. If you miss no more than one of these nine self-assessment questions, you might want to move ahead to the section "Exam Preparation Tasks." Table 17-1 lists the major headings in this chapter and the "Do I Know This Already?" quiz questions covering the material in those headings so that you can assess your knowledge of these specific areas. The answers to the "Do I Know This Already?" quiz appear in Appendix A.

Table 17-1 "Do I Know This Already?" Foundation Topics Section-to-Question Mapping

Foundation Topics Section


Global Unicast Addressing, Routing, and Subnetting

1, 2

IPv6 Protocols and Addressing


Configuring IPv6 Routing and Routing Protocols


IPv6 Transition Options


  1. Which of the following is the most likely organization from which an enterprise could obtain an administrative assignment of a block of IPv6 global unicast IP addresses?

    1. An ISP

    2. ICANN

    3. An RIR

    4. Global unicast addresses are not administratively assigned by an outside organization.

  2. Which of the following is the shortest valid abbreviation for FE80:0000:0000:0100:0000:0000:0000:0123?

    1. FE80::100::123

    2. FE8::1::123

    3. FE80::100:0:0:0:123:4567

    4. FE80:0:0:100::123

  3. Which of the following answers lists a multicast IPv6 address?

    1. 2000::1:1234:5678:9ABC

    2. FD80::1:1234:5678:9ABC

    3. FE80::1:1234:5678:9ABC

    4. FF80::1:1234:5678:9ABC

  4. Which of the following answers list either a protocol or function that can be used by a host to dynamically learn its own IPv6 address?

    1. Stateful DHCP

    2. Stateless DHCP

    3. Stateless autoconfiguration

    4. Neighbor Discovery Protocol

  5. Which of the following help allow an IPv6 host to learn the IP address of a default gateway on its subnet?

    1. Stateful DHCP

    2. Stateless RS

    3. Stateless autoconfiguration

    4. Neighbor Discovery Protocol

  6. Which of the following are routing protocols that support IPv6?

    1. RIPng

    2. RIP-2

    3. OSPFv2

    4. OSPFv3

    5. OSPFv4

  7. In the following configuration, this router's Fa0/0 interface has a MAC address of 4444.4444.4444. Which of the following IPv6 addresses will the interface use?

  8. ipv6 unicast-routing
    ipv6 router rip tag1
    interface FastEthernet0/0
     ipv6 address 3456::1/64
    1. 3456::C444:44FF:FE44:4444

    2. 3456::4444:44FF:FE44:4444

    3. 3456::1

    4. FE80::1

    5. FE80::6444:44FF:FE44:4444

    6. FE80::4444:4444:4444

  9. In the configuration text in the previous question, RIP was not working on interface Fa0/0. Which of the following configuration commands would enable RIP on Fa0/0?

    1. network 3456::/64

    2. network 3456::/16

    3. network 3456::1/128

    4. ipv6 rip enable

    5. ipv6 rip tag1 enable

  10. Which of the following IPv4-to-IPv6 transition methods allows an IPv4-only host to communicate with an IPv6-only host?

    1. Dual-stack

    2. 6to4 tunneling

    3. ISATAP tunneling

    4. NAT-PT

Foundation Topics

The world has changed tremendously over the last 10–20 years as a result of the growth and maturation of the Internet and networking technologies in general. Twenty years ago, no global network existed to which the general populace could easily connect. Ten years ago, the public Internet had grown to the point where people in most parts of the world could connect to the Internet, but with most Internet users being the more computer-savvy people. Today, practically everyone seems to have access, through their PCs, handheld devices, phones, or even the refrigerator.

The eventual migration to IPv6 will likely be driven by the need for more addresses. Practically every mobile phone supports Internet traffic, requiring the use of an IP address. Most new cars have the ability to acquire and use an IP address, along with wireless communications, allowing the car dealer to contact the customer when the car's diagnostics detect a problem with the car. Some manufacturers have embraced the idea that all their appliances need to be IP enabled.

Besides the sheer growth in the need for IPv4 addresses, edicts from governmental agencies could drive demand for IPv6. As of this writing, the U.S. government had set a date in 2008 by which all government agencies should be running IPv6 in their core IP networks. Such initiatives can help drive adoption of IPv6.

While the two biggest reasons why networks might migrate to IPv6 are the need for more addresses and mandates from government organizations, at least IPv6 includes some attractive features and migration tools. Some of those advantages are as follows:

  • Address assignment features: IPv6 address assignment allows easier renumbering, dynamic allocation, and recovery of addresses, with nice features for mobile devices to move around and keep their IP address (thereby avoiding having to close and reopen an application).

  • Aggregation: IPv6's huge address space makes for much easier aggregation of blocks of addresses in the Internet.

  • No need for NAT/PAT: Using publicly registered unique addresses on all devices removes the need for NAT/PAT, which also avoids some of the application layer and VPN-tunneling issues caused by NAT.

  • IPsec: IPsec works with both IPv4 and IPv6, but it is required on IPv6 hosts, so you can rely on support for IPsec as needed for VPN tunneling.

  • Header improvements: While it might seem like a small issue, the IPv6 header improves several things compared to IPv4. In particular, routers do not need to recalculate a header checksum for every packet, reducing per-packet overhead. Additionally, the header includes a flow label that allows easy identification of packets sent over the same single TCP or User Datagram Protocol (UDP) connection.

  • Transition tools: As is covered in the last major section of this chapter, IPv6 has many tools to help with the transition from IPv4 to IPv6.

The worldwide migration from IPv4 to IPv6 will not be an event, or even a year on the calendar. Rather, it will be a long process, a process that has already begun. Network engineers have a growing need to learn more about IPv6. This chapter covers the basics of IPv6, ending with some discussions about the issues of living in a world in which both IPv4 and IPv6 will likely coexist for quite a long time.

NOTE - Information Week ( published an interesting article about the need to migrate to IPv6, around the time this book was being completed. To see the article, search the website for the article "The Impending Internet Address Shortage."

Global Unicast Addressing, Routing, and Subnetting

One of the original design goals for the Internet was that all organizations would register and be assigned one or more public IP networks (Class A, B, or C). By registering to use a particular public network number, the company or organization using that network was assured by the numbering authorities that no other company or organization in the world would be using the addresses in that network. As a result, all hosts in the world would have globally unique IP addresses.

From the perspective of the Internet infrastructure, in particular the goal of keeping Internet routers' routing tables from getting too large, assigning an entire network to each organization helped to some degree. The Internet routers could ignore all subnets, instead having a route for each classful network. For example, if a company registered and was assigned Class B network, the Internet routers just needed one route for that entire network.

Over time, the Internet grew tremendously. It became clear by the early 1990s that something had to be done, or the growth of the Internet would grind to a halt when all the public IP networks were assigned, and no more existed. Additionally, the IP routing tables in Internet routers were becoming too large for the router technology of that day. So, the Internet community worked together to come up with both some short-term and long-term solutions to two problems: the shortage of public addresses and the size of the routing tables.

The short-term solutions included a much smarter public address assignment policy, where public addresses were not assigned as only Class A, B, and C networks, but as smaller subdivisions (prefixes), reducing waste. Additionally, the growth of the Internet routing tables was reduced by smarter assignment of the address ranges. For example, assigning the Class C networks that begin with 198 to only a particular Internet service provider (ISP) in a particular part of the world allowed other ISPs to use one route for—in other words, all addresses that begin with 198—rather than a route for each of the 65,536 different Class C networks that begin with 198. Finally, NAT/PAT achieved amazing results by allowing a typical home or small office to consume only one public IPv4 address, greatly reducing the need for public IPv4 addresses.

The ultimate solution to both problems is IPv6. The sheer number of IPv6 addresses takes care of the issue of running out of addresses. The address assignment policies already used with IPv4 have been refined and applied to IPv6, with good results for keeping the size of IPv6 routing tables smaller in Internet routers. The following sections provide a general discussion of both issues, in particular how global unicast addresses, along with good administrative choices for how to assign IPv6 address prefixes, aid in routing in the global Internet. These sections conclude with a discussion of subnetting in IPv6.

Global Route Aggregation for Efficient Routing

By the time IPv6 was being defined in the early 1990s, it was clear that thoughtful choices about how to assign the public IPv4 address space could help with the efficiency of Internet routers by keeping their routing tables much smaller. By following those same well-earned lessons, IPv6 public IP address assignment can make for even more efficient routing as the Internet migrates to IPv6.

The address assignment strategy for IPv6 is elegant, but simple, and can be roughly summarized as follows:

  • Public IPv6 addresses are grouped (numerically) by major geographic region.

  • Inside each region, the address space is further subdivided by ISP inside that region.

  • Inside each ISP in a region, the address space is further subdivided for each customer.

The same organizations handle this address assignment for IPv6 as for IPv4. The Internet Corporation for Assigned Network Numbers (ICANN, owns the process. ICANN assigns one or more IPv6 address ranges to each Regional Internet Registry (RIR), of which five exist at the time of publication, roughly covering North America, Central/South America, Europe, Asia/Pacific, and Africa. These RIRs then subdivide their assigned address space into smaller portions, assigning prefixes to different ISPs and other smaller registries, with the ISPs then assigning even smaller ranges of addresses to their customers.

NOTE - The Internet Assigned Numbers Authority (IANA) formerly owned the address assignment process, but it was transitioned to ICANN.

The IPv6 global address assignment plan results in more efficient routing, as shown in Figure 17-1. The figure shows a fictitious company (Company1) that has been assigned an IPv6 prefix by a fictitious ISP, NA-ISP1 (standing for North American ISP number 1). The figure lists the American Registry for Internet Numbers (ARIN), which is the RIR for North America.

Figure 17-1

Conceptual View of IPv6 Global Routes

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