Chapter 17: IP Version 6

Cisco Press

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In the figure, the IPv6-based PC1 sends an IPv6 packet. Router R1 then encapsulates or tunnels the IPv6 packet into a new IPv4 header, with a destination IPv4 address of an address on Router R4. Routers R2 and R3 happily forward the packet, because it has a normal IPv4 header, while R4 de-encapsulates the original IPv6 packet, forwarding it to IPv6-based PC2. It's called a tunnel in part because the IPv6 packets inside the tunnel can't be seen while traversing the tunnel; the routers in the middle of the network, R2 and R3 in this case, perceive the packets as IPv4 packets.

Several types of IPv6-to-IPv4 tunnels exist. To perform the tunneling shown by the routers in Figure 17-11, the first three of the following types of tunnels could be used, with the fourth type (Teredo tunnels) being used by hosts:

  • Manually configured tunnels (MCT): A simple configuration in which tunnel interfaces, a type of virtual router interface, are created, with the configuration referencing the IPv4 addresses used in the IPv4 header that encapsulates the IPv6 packet.

  • Dynamic 6to4 tunnels: This term refers to a specific type of dynamically created tunnel, typically done on the IPv4 Internet, in which the IPv4 addresses of the tunnel endpoints can be dynamically found based on the destination IPv6 address.

  • Intra-site Automatic Tunnel Addressing Protocol (ISATAP): Another dynamic tunneling method, typically used inside an enterprise. Unlike 6to4 tunnels, ISATAP tunnels do not work if IPv4 NAT is used between the tunnel endpoints.

  • Teredo tunneling: This method allows dual-stack hosts to create a tunnel to another host, with the host itself both creating the IPv6 packet and encapsulating the packet inside an IPv4 header.

Figure 17-12 shows the basic idea behind the Teredo tunnel.

Figure 17-12

Example Encapsulation for a Teredo Host-Host Tunnel

Translating Between IPv4 and IPv6 with NAT-PT

Both classes of IPv6 transition features mentioned so far in this chapter, dual stack and tunnels, rely on the end hosts to at least support IPv6, if not both IPv4 and IPv6. However, in some cases, an IPv4-only host needs to communicate with an IPv6-only host. A third class of transition features needs to be used in this case: a tool that translates the headers of an IPv6 packet to look like an IPv4 packet, and vice versa.

In Cisco routers, Network Address Translation–Protocol Translation (NAT-PT), defined in RFC 2766, can be used to perform the translation. To do its work, a router configured with NAT-PT must know what IPv6 address to translate to which IPv4 address and vice versa, the same kind of information held in the traditional NAT translation table. And like traditional NAT, NAT-PT allows static definition, dynamic NAT, and dynamic PAT, which can be used to conserve IPv4 addresses.

Transition Summary

Table 17-11 summarizes the transition options for IPv6 for easier reference and study.

Table 17-11 Summary of IPv6 Transition Options


Particular Type


Dual stack

Supports both protocols, and sends IPv4 to IPv4 hosts and IPv6 to IPv6 hosts



Tunnel is manually configured; sends IPv6 through IPv4 network, typically between routers



Tunnel endpoints are dynamically discovered; sends IPv6 through IPv4 network, typically between routers



Tunnel endpoints are dynamically discovered; sends IPv6 through IPv4 network between routers; does not support IPv4 NAT



Typically used by hosts; host creates IPv6 packet and encapsulates in IPv4


Router translates between IPv4 and IPv6; allows IPv4 hosts to communicate with IPv6 hosts

Exam Preparation Tasks

Review All the Key Topics

Review the most important topics from this chapter, noted with the Key Topics icon in the outer margin of the page. Table 17-12 lists a reference of these key topics and the page numbers on which each is found.

Table 17-12 Key Topics for Chapter 17

Key Topic Element


Page Number

Figure 17-1

Route aggregation concepts in the global IPv6 Internet



Rules for abbreviating IPv6 addresses



Rules for writing IPv6 prefixes


Figure 17-3

Example prefix assignment process



Major steps in subdividing a prefix into a subnet prefix in an enterprise


Figure 17-5

Example and structure of IPv6 subnets


Figure 17-7

Structure of IPv6 addresses and EUI-64 formatted interface ID


Table 17-6

List of four main options to IPv6 address configuration


Table 17-7

Comparisons of IPv6 stateful and stateless DHCP services



Different types and purposes of IPv6 addresses


Figure 17-10

Format and structure of link local addresses



Summary of the steps a host takes to learn its address, prefix length, DNS, and default router


Table 17-9

Summary of prefixes and purpose of most common types of IPv6 addresses



Configuration checklist for IPv6 configuration


Table 17-11

List of IPv6 transition options


Complete the Tables and Lists from Memory

Print a copy of Appendix J, "Memory Tables," (found on the CD) or at least the section for this chapter, and complete the tables and lists from memory. Appendix K, "Memory Tables Answer Key," also on the CD, includes completed tables and lists to check your work.

Definitions of Key Terms

Define the following key terms from this chapter, and check your answers in the glossary:

Dual stacks, global unicast address, ISP prefix, link local address, NAT-PT, Neighbor Discovery Protocol (NDP), Regional Internet Registry (RIR), registry prefix, site prefix, stateful DHCP, stateless autoconfiguration, stateless DHCP, subnet prefix, unique local address

Command Reference to Check Your Memory

While you should not necessarily memorize the information in the tables in this section, this section does include a reference for the configuration and EXEC commands covered in this chapter. Practically speaking, you should memorize the commands as a side effect of reading the chapter and doing all the activities in this exam preparation section. To check to see how well you have memorized the commands as a side effect of your other studies, cover the left side of the table with a piece of paper, read the descriptions on the right side, and see whether you remember the command.

Table 17-13 Chapter 17 Configuration Command Reference



ipv6 unicast-routing

Global command that enables IPv6 routing on the router

ipv6 router rip tag

Global command that enables RIPng

ipv6 rip name enable

Interface subcommand that enables RIPng on the interface

ipv6 address {ipv6-address/prefix-length | prefix-name sub-bits/prefix-length} eui-64

Interface subcommand that manually configures either the entire interface IP address, or a /64 prefix with the router building the EUI-64 format interface ID automatically

ipv6 host name ipv6-address1 [ipv6-address2...ipv6-address4]

Global command to create a static host name definition

ip name-server server-address1 [server-address2...server-address6]

Global command to point to one or more name servers, to resolve a name into either an IPv4 or IPv6 address

[no] ip domain-lookup

Global command that enables the router as a DNS client, or with the no option, disables the router as a DNS client

Table 17-14 Chapter 17 EXEC Command Reference



show ipv6 route

Lists IPv6 routes

show ipv6 route ip-address

Lists the route(s) this router would match for packets sent to the listed address

show ipv6 route [prefix/prefix-length]

Lists the route for the specifically listed prefix/length

show ipv6 interface [type number]

Lists IPv6 settings on an interface, including link local and other unicast IP addresses

show ipv6 interface brief

Lists interface status and IPv6 addresses for each interface

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