- The primary purpose of the core IPv6 protocol mirrors the same purpose of the IPv4 protocol. That core IPv6 protocol, as defined in RFC 2460, defines a packet concept, addresses for those packets, and the role of hosts and routers. These rules enable the devices to forward packets sourced by hosts, through multiple routers, so that they arrive at the correct destination host.
- However, because IPv6 impacts so many other functions in a TCP/IP network, many more RFCs must define details of IPv6. Some other RFCs define how to migrate from IPv4 to IPv6. Others define new versions of familiar protocols or replace old protocols with new ones. For example:
- Older OSPF Version 2 Upgraded to OSPF Version 3: The older OSPF version 2 works for IPv4 but not for IPv6, so a newer version, OSPF version 3, was created to support IPv6.
- ICMP Upgraded to ICMP Version 6: ICMP worked well with IPv4 but needed to be changed to support IPv6. The new name is ICMPv6.
- ARP Replaced by Neighbor Discovery Protocol: For IPv4, ARP discovers the MAC address used by neighbors. IPv6 replaces ARP with a more general Neighbor Discovery Protocol (NDP).
- Although the term IPv6, when used broadly, includes many protocols, the one specific protocol called IPv6 defines the new 128-bit IPv6 address.
- As with many functions of IPv6, IPv6 routing looks just like IPv4 routing from a general perspective, with the differences being clear only when you look at the specifics. IPv6 uses these ideas the same way as IPv4:
- To be able to build and send IPv6 packets out an interface, end-user devices need an IPv6 address on that interface.
- End-user hosts need to know the IPv6 address of a default router, to which the host sends IPv6 packets if the host is in a different subnet.
- IPv6 routers deencapsulate and reencapsulate each IPv6 packet when routing the packet.
- IPv6 routers make routing decisions by comparing the IPv6 packet’s destination address to the router’s IPv6 routing table; the matched route lists directions of where to send the IPv6 packet next.
- IPv6 uses a convenient hexadecimal (hex) format for addresses. To make it more readable, IPv6 uses a format with 8 sets of 4 hex digits, with each set of 4 digits separated by a colon. For example:
- Two basic rules let you, or any computer, shorten or abbreviate an IPv6 address:
- Inside each quartet of four hex digits, remove the leading 0s (0s on the left side of the quartet) in the three positions on the left. (Note: At this step, a quartet of 0000 will leave a single 0.)
- Find any string of two or more consecutive quartets of all hex 0s, and replace that set of quartets with a double colon (::). The :: means “two or more quartets of all 0s.” However, you can use :: only once in a single address, because otherwise the exact IPv6 might not be clear.
- To expand an IPv6 address back into its full unabbreviated 32-digit number, use two similar rules. The rules basically reverse the logic of the previous two rules.
- In each quartet, add leading 0s as needed until the quartet has four hex digits.
- If a double colon (::) exists, count the quartets currently shown; the total should be less than 8. Replace the :: with multiple quartets of 0000 so that 8 total quartets exist.
- IPv6 uses a mask concept, called the prefix length, similar to IPv4 subnet masks. Similar to the IPv4 prefix-style mask, the IPv6 prefix length is written as a / followed by a decimal number. The prefix length defines how many bits of the IPv6 address defines the IPv6 prefix, which is basically the same concept as the IPv4 subnet ID.
- Like IPv4, you can start with an IPv6 address and prefix length and find the prefix, with the same general rules that you use in IPv4. If the prefix length is /P, then use these rules:
- Copy the first P bits.
- Change the rest of the bits to 0.
- When using a prefix length that happens to be a multiple of 4, you do not have to think in terms of bits but in terms of hex digits. A prefix length that is a multiple of 4 means that each hex digit is either copied or changed to 0. Just for completeness, if the prefix length is indeed a multiple of 4, the process becomes
- Identify the number of hex digits in the prefix by dividing the prefix length (which is in bits) by 4.
- Copy the hex digits determined to be in the prefix per the first step.
- Change the rest of the hex digits to 0.
Answer these review questions. You can find the answers at the bottom of the last page of the chapter. For thorough explanations, see DVD Appendix C, “Answers to Review Questions.”
- Which of the following was a short-term solution to the IPv4 address exhaustion problem?
- IP version 6
- IP version 5
- A router receives an Ethernet frame that holds an IPv6 packet. The router then makes a decision to route the packet out a serial link. Which of the following statements is true about how a router forwards an IPv6 packet?
- The router discards the Ethernet data link header and trailer of the received frame.
- The router makes the forwarding decision based on the packet’s source IPv6 address.
- The router keeps the Ethernet header, encapsulating the entire frame inside a new IPv6 packet before sending it over the serial link.
- The router uses the IPv4 routing table when choosing where to forward the packet.
- Which of the following is the shortest valid abbreviation for FE80:0000:0000:0100:0000:0000:0000:0123?
- Which of the following is the shortest valid abbreviation for 2000:0300:0040:0005:6000:0700:0080:0009?
- Which of the following is the unabbreviated version of IPv6 address 2001:DB8::200:28?
- Which of the following is the prefix for address 2000:0000:0000:0005:6000:0700:0080:0009, assuming a mask of /64?
Review All the Key Topics
Review the most important topics from this chapter, noted with the Key Topic icon. Table 25-6 lists these key topics and where each is discussed.
Table 25-6.Key Topics for Chapter 25
Key Topic Element
Similarities between IPv4 and IPv6
Rules for abbreviating IPv6 addresses
Rules for expanding an abbreviated IPv6 address
Process steps to find an IPv6 prefix, based on the IPv6 address and prefix length
Complete the Tables and Lists from Memory
Print a copy of DVD Appendix M, “Memory Tables,” or at least the section for this chapter, and complete the tables and lists from memory. DVD Appendix N, “Memory Tables Answer Key,” includes completed tables and lists for you to check your work.
Definitions of Key Terms
After your first reading of the chapter, try to define these key terms, but do not be concerned about getting them all correct at that time. Chapter 30 directs you in how to use these terms for late-stage preparation for the exam.
IPv4 address exhaustion
IP version 6 (IPv6)
OSPF version 3 (OSPFv3)
EIGRP version 6 (EIGRPv6)
Additional Practice with IPv6 Address Abbreviations
For additional practice abbreviating IPv6 addresses:
- DVD Appendix K, “Practice for Chapter 25: Fundamentals of IP Version 6,” has some additional practice problems listed.
- Create your own problems using any real router or simulator. Get into the router CLI, into configuration mode, and configure a 32-digit unabbreviated IPv6 address. Then predict the shortest abbreviation. Finally, use the show ipv6 interface command to see if the router used the same abbreviation you used.