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So far, this chapter has focused on the IPv6 addresses that most closely match the concept
of IPv4 addresses: the global unicast IPv6 address. This section now takes a broader look
at IPv6 addressing, including some concepts that can be tied to older IPv4 concepts, and
some that are unique to IPv6.
This section begins with a brief overview of IPv6 addressing. It then looks at unicast IPv6
addresses, along with a brief look at some of the commonly used multicast addresses.
This section ends with a discussion of a couple of related protocols, namely Neighbor Discovery
Protocol (NDP) and Duplicate Address Detection (DAD).
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Table 16-7 Comparing Stateless and Stateful DHCPv6 Services
Feature Stateful
DHCP
Stateless
DHCP
Remembers IPv6 address (state information) of clients that
make requests
Yes No
Assigns IPv6 address to client Yes No
Supplies useful information, such as DNS server IP addresses Yes Yes
Most useful in conjunction with stateless autoconfiguration No Yes
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Overview of IPv6 Addressing
The whole concept of global unicast addressing does have many similarities as compared
with IPv4. If viewing IPv4 addresses from a classless perspective, both IPv4 and IPv6
global unicast addresses have two parts: subnet plus host for IPv4 and prefix plus interface
ID for IPv6. The format of the addresses commonly list a slash followed by the prefix
length–a convention sometimes referred to as CIDR notation, and other times as prefix
notation. Subnetting works much the same, with a public prefix assigned by some numbering
authority, and the Enterprise choosing subnet numbers, extending the length of the
prefix to make room to number the subnets.
IPv6 addressing, however, includes several other types of unicast IPv6 addresses beside
the global unicast address. Additionally, IPv6 defines other general categories of addresses,
as summarized in this list.
■ Unicast: Like IPv4, hosts and routers assign these IP addresses to a single interface
for the purpose of allowing that one host or interface to send and receive IP packets.
■ Multicast: Like IPv4, these addresses represent a dynamic group of hosts, allowing a
host to send one packet that is then delivered to every host in the multicast group.
IPv6 defines some special-purpose multicast addresses for overhead functions (such
as NDP). IPv6 also defines ranges of multicast addresses for application use.
■ Anycast: This address type allows the implementation of a nearest server among duplicate
servers concept. This design choice allows servers that support the exact same
function to use the exact same unicast IP address. The routers then forward a packet
destined for such an address to the nearest server that is using the address.
Two big differences exist when comparing general address categories for IPv4 and IPv6.
First, IPv6 adds the formal concept of Anycast IPv6 addresses as shown in the preceding
list. IPv4 does not formally define an Anycast IP address concept, although a similar concept
may be implemented in practice. Second, IPv6 simply has no Layer 3 broadcast addresses.
For example, all IPv6 routing protocols send Updates either to Unicast or
Multicast IPv6 addresses, and overhead protocols such as NDP make use of multicasts as
well. In IPv4, ARP still uses broadcasts, and older routing protocols such as RIP-1 also
used broadcasts. With IPv6, there is no need to calculate a subnet broadcast address
(hoorah!) and no need to make hosts process overhead broadcast packets meant only for a
few devices in a subnet.
Finally, note that IPv6 hosts and router interfaces typically have at least two IPv6 addresses
and may well have more. Hosts and routers typically have a Link Local type of
IPv6 address (as described in the upcoming section “Link Local Unicast Addresses”). A
router may or may not have a global unicast address, and may well have multiple. IPv6
simply allows the configuration of multiple IPv6 addresses with no need for or concept of
secondary IP addressing.
Unicast IPv6 Addresses
IPv6 supports three main types of unicast addresses: link local, global unicast, and unique
local. This section takes a brief look at link local and unique local addresses.
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Chapter 16 : IP Version 6 Addressing 551
Unique Local IPv6 Addresses
Unique local unicast IPv6 addresses have the same function as IPv4 RFC 1918 private addresses.
RFC 4193 states that these addresses should be used inside a private organization,
and should not be advertised into the Internet. Unique local unicast addresses begin
with hex FD (FD00::/8), with the format shown in Figure 16-9.
To use these addresses, an Enterprise engineer would choose a 40-bit global ID in a
pseudo-random manner rather than asking for a registered public prefix from an ISP or
other registry. To form the complete prefix, the chosen 40 bits would be combined with
the initial required 8 bits (hex FD) to form a 48-bit site prefix. The engineer can then use a
16-bit subnet field to create subnets, leaving a 64-bit Interface ID. The interface ID could
be created by static configuration or by the EUI-64 calculation.
This type of unicast address gives the engineer the ability to create the equivalent of an
IPv4 private address structure, but given the huge number of available public IPv6 addresses,
it may be more likely that engineers plan to use global unicast IP addresses
throughout an Enterprise.
Link Local Unicast Addresses
IPv6 uses link local addresses for sending and receiving IPv6 packets on a single subnet.
Many such uses exist; here’s just a small sample:
■ Used as the source address for RS and RA messages for router discovery (as previously
shown in Figure 16-7)
■ Used by Neighbor discovery (the equivalent of ARP for IPv6)
■ As the next-hop IPv6 address for IP routes
By definition, routers use a link local scope for packets sent to a link local IPv6 address.
The term link local scope means exactly that–the packet should not leave the local link, or
local subnet if you will. When a router receives a packet destined for such a destination
address, the router does not forward the packet.
The link local IPv6 addresses also help solve some chicken-and-egg problems because
each host, router interface, or other device can calculate its own link local IPv6 address
without needing to communicate with any other device. So, before sending the first packets,
the host can calculate its own link local address, so the host has an IPv6 address to
use when doing its first overhead messages. For example, before a host sends an NDP RS
Subnet Prefix
8 Bits 40 Bits
FD
16 Bits 64 Bits
Global ID Interface ID
(Pseudo-Random)
Subnet
Figure 16-9 Unique Local Address Format
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10 Bits
FE80/10
1111111010 All 0s
64 Bits
Interface ID
54 Bits
Figure 16-10 Link Local Address Format
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Table 16-8 Common Link-Local Multicast Addresses
Type of Address Purpose Prefix Easily Seen Hex Prefix(es)
Global unicast Unicast packets sent
through the public Internet
2000::/3 2 or 3
Unique local Unicast packets inside
one organization
FD00::/8 FD
(router solicitation) message, the host will have already calculated its link local address,
which can be used as the source IPv6 address on the RS message.
Link local addresses come from the FE80::/10 range, meaning the first 10 bits must be
1111 1110 10. An easier range to remember is that all hex link local addresses begin FE8,
FE9, FEA, or FEB. However, practically speaking, for link local addresses formed automatically
by a host (rather than through static configuration), the address always starts
FE80, because the automatic process sets bits 11-64 to binary 0s. Figure 16-10 shows the
format of the link local address format under the assumption that the host or router is deriving
its own link local address, therefore using 54 binary 0s after the FE80::/10 prefix.
IPv6 Unicast Address Summary
You may come across a few other types of IPv6 addresses in other reading. For example,
earlier IPv6 RFCs defined the Site Local address type, which was meant to be used like
IPv4 private addresses. However, this address type has been deprecated (RFC 3879). Also,
the IPv6 migration and coexistence tools discussed in Chapter 18 use some conventions
for IPv6 unicast addresses such that IPv4 addresses are imbedded in the IPv6 address.
Additionally, it is helpful to know about other special unicast addresses. An address of all
hex 0s, written ::/128, represents an unknown address. This can be used as a source IPv6
address in packets when a host has no suitable IPv6 address to use. The address ::1/128,
representing an address of all hex 0s except a final hex digit 1, is a loopback address. Packets
sent to this address will be looped back up the TCP/IP stack, allowing for easier software
testing. (This is the equivalent of IPv4’s 127.0.0.1 loopback address.)
Table 16-8 summarizes the IPv6 unicast address types for easier study.
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Table 16-8 Common Link-Local Multicast Addresses
Type of Address Purpose Prefix Easily Seen Hex Prefix(es)
Link local Packets sent in the
local subnet
FE80::/10 FE8
Site local Deprecated; originally
meant to be used like
private IPv4 addresses
FECO::/10 FEC, FED, FEE, FEF
Unspecified An address used when
a host has no usable
IPv6 address
::/128 N/A
Loopback Used for software
testing, like IPv4’s
127.0.0.1
::1/128 N/A
IPv6 RFCs define the FE80::/10 prefix, which technically means that the first three hex
digits could be FE8, FE9, FEA, or FEB. However, bit positions 11-64 of link local addresses
should be 0, so in practice, link local addresses should always begin with FE80.
Table 16-9 Common Multicast Addresses
Purpose IPv6 Address IPv4 Equivalent
All IPv6 nodes on the link FF02::1 subnet broadcast address
Multicast and Other Special IPv6 Addresses
IPv6 supports multicasts on behalf of applications and multicasts to support the inner
workings of IPv6. To aid this process, IPv6 defines ranges of IPv6 addresses and an associated
scope, with the scope defining how far away from the source of the packet that the
network should forward a multicast.
All IPv6 multicast addresses begin with FF::/8 – in other words, with FF as the first two
digits. Multicasts with a link local scope, like most of the multicast addresses referenced
in this chapter, begin with FF02::/16; the 2 in the fourth hex digit identifies the scope as
link local. A fourth digit of hex 5 identifies the broadcast as site local scope, with those
multicasts beginning with FF05::/16.
For reference, Table 16-9 lists some of the more commonly seen IPv6 multicast addresses.
Of particular interest are the addresses chosen for use by RIP, OSPF, and EIGRP, which
somewhat mirror the multicast addresses each protocol uses for IPv4. Note also that all
but the last two entries have link local scope.
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Layer 2 Addressing Mapping and Duplicate Address Detection
As with IPv4, any device running IPv6 needs to determine the data link layer address used
by devices on the same link. IPv4 uses Address Resolution Protocol (ARP) on LANs and
Inverse ARP (InARP) on Frame Relay. IPv6 defines a couple of new protocols that perform
the same function. These new functions use ICMPv6 messages and avoid the use of
broadcasts, in keeping with IPv6’s avoidance of broadcasts. This section gives a brief explanation
of each protocol.
Neighbor Discovery Protocol for Layer 2 Mapping
When an IPv6 host or router needs to send a packet to another host or router on the same
LAN, the host/router first looks in its neighbor database. This database contains a list of
all neighboring IPv6 addresses (addresses in connected links) and their corresponding
MAC addresses. If not found, the host or router uses the Neighbor Discovery Protocol
(NDP) to dynamically discover the MAC address.
Figure 16-11 shows a sample of such a process, using the same host and router seen earlier
in Figure 16-8.
The process acts like the IPv4 ARP process, just with different details. In this case, PC1
sends a multicast message called a Neighbor Solicitation (NS) ICMP message, asking R1
to reply with R1’s MAC address. R1 sends a Neighbor Advertisement (NA) ICMP message,
unicast back to PC1, listing B’s MAC address. Now PC1 can build a data link frame with
R1’s MAC listed as the destination address and send encapsulated packets to R1.
The NS message uses a special multicast destination address called a solicited node multicast
address. On any given link, the solicited node multicast address represents all hosts
with the same last 24 bits of their IPv6 addresses. By sending packets to the solicited node
multicast address, the packet reaches the correct host, but it may also reach a few other
hosts–which is fine. (Note that packets sent to a solicited node multicast address have a
link local scope.)
The solicited node multicast address begins with FF02::1:FF:0/104. The final 24 bits (6 hex
digits) of the address is formed by adding the last 24 bits of the IPv6 address to which the
message is being sent. This convention allows convenient use of LAN multicast addresses,
which begin with hex 01005E hex, followed by one additional binary 0, and then an addi-
All IPv6 routers on the link FF02::2 N/A
OSPF messages FF02::5, FF02::6 224.0.0.5, 224.0.0.6
RIP-2 messages FF02::9 224.0.0.9
EIGRP messages FF02::A 224.0.0.10
DHCP relay agents (routers that
forward to the DHCP server)
FF02:1:2 N/A
DHCP servers (site scope) FF05::1:3 N/A
All NTP servers (site scope) FF05::101 N/A
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Source = PC1 IPv6 Address
Dest = Solicited Node Mcast of R1
Question = What’s Your Datalink Address?
PC1 R1
Neighbor Solicitation
Source = R1’s IPv6 Address
Dest = PC1’s IPv6 Address
Answer = MAC 0013.197B.5004
Neighbor Advertisement
Figure 16-11 Neighbor Discovery Protocol
tional 23 bits–in this case taken from the low order 23 bits of the IPv6 address. All IPv6
listen for frames sent to their own solicited node multicast address, so that when a host or
router receives such a multicast, the host can realize that it should reply. For example, in
this case, based on R1’s IPv6 address previously seen in Figure 16-8:
■ R1’s IPv6 address: 2340:1111:AAAA:1:213:19FF:FE7B:5004
■ R1’s solicited node address: FF02::1:FF:7B:5004
Note: The corresponding Ethernet multicast MAC address would be 0100.5E7B.5004.
Duplicate Address Detection (DAD)
When an IPv6 interface first learns an IPv6 address, or when the interface begins working
after being down for any reason, the interface performs duplicate address detection
(DAD). The purpose of this check is to prevent hosts from creating problems by trying to
use the same IPv6 address already used by some other host on the link.
To perform such a function, the interface uses the same NS message shown in Figure 16-
11 but with small changes. To check its own IPv6 address, a host sends the NS message to
the solicited node multicast address based on its own IPv6 address. If some host sends a
reply, listing the same IPv6 address as the source address, the original host has found that
a duplicate address exists.
Inverse Neighbor Discovery
The ND protocol discussed in this section starts with a known neighbor’s IPv6 address
and seeks to discover the link layer address used by that IPv6 address. On Frame Relay
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Table 16-10 Router IOS IPv6 Configuration Command Reference
Command Description
ipv6 address address/length Static configuration of the entire IPv6 unicast address.
ipv6 address prefix/length eui-64 Static configuration of the first 64 address bits; the
router derives the last 64 bits with EUI-64.
networks, and with some other WAN data link protocols, the order of discovery is reversed.
A router begins with knowledge of the neighbor’s link layer address and instead
needs to dynamically learn the IPv6 address used by that neighbor.
IPv4 solves this discovery problem on LANs using ARP, and the reverse problem over
Frame Relay using Inverse ARP (InARP). IPv6 solves the problem on LANs using ND, and
now for Frame Relay, IPv6 solves this problem using Inverse Neighbor Discovery (IND).
IND, also part of the ICMPv6 protocol suite, defines an Inverse NS (INS) and Inverse NA
(INA) message. The INS message lists the known neighbor link layer address (DLCI for
Frame Relay), and the INS asks for that neighboring device’s IPv6 addresses. The details
inside the INS message include the following:
■ Source IPv6: IPv6 unicast of sender
■ Destination IPv6: FF02::1 (all IPv6 hosts multicast)
■ Link layer addresses
■ Request: Please reply with your IPv6 address(es)
The IND reply lists all the IPv6 addresses. As with IPv4, the show frame-relay map command
lists the mapping learned from this process.
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