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This section still focuses on global unicast IPv6 addresses but now examines the topic of
how a host, router interface, or other device knows what global unicast IPv6 address to
use. Also, hosts (and sometimes routers) also need to know a few other facts that can be
learned at the same time as learning their IPv6 address. So, this section also discusses how
hosts can get all the following relevant information that lets them use their global unicast
addresses:
■ IP address
■ IP subnet mask (prefix length)
■ Default router IP address
■ DNS IP address(es)
IPv6 actually has four major options for IPv6 global unicast address assignment. This section
looks at these options in the same order as listed in Table 16-5. Each method can use
dynamic processes or static configuration, and each method can differ in terms of how a
host or router gathers the other pertinent information (such as DNS IP addresses). Table
16-5 summarizes these main methods for easier review.
The rest of this section develops more detail about the topics in the table. Some of the
processes work much like IPv4, and some do not. Regardless, as you work through the
Table 16-5 Summary of IPv6 Address Assignment for Global Unicast Addresses
Method Dynamic
or static
Prefix and
length learned
from...
Host
learned
from...
Default router
learned from...
DNS
addresses
learned from...
Stateful
DHCP
Dynamic DHCP Server DHCP
Server
Router, using
NDP
(Stateful) DHCP
Server
Stateless
autoconfig
Dynamic Router, using
NDP
Derived
from
MAC
Router, using
NDP
Stateless DHCP
Static configuration
Static Local config Local config
Router, using
NDP
Stateless DHCP
Static config
with
EUI-64
Static Local config Derived
from
MAC
Router, using
NDP
Stateless DHCP
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Chapter 16 : IP Version 6 Addressing 545
material, keep in mind one key fact about how IPv6 protocols approach the address assignment
process:
IPv6 address assignment processes may split the IPv6 address assignment into two
parts: the prefix/length assignment and the host (interface ID) assignment.
Stateful DHCP for IPv6
IPv6 hosts can use stateful DHCP to learn and lease an IP address and corresponding prefix
length (mask), the IP address of the default router, and the DNS IP address(es). The
concept works basically like DHCP for IPv4; the host sends a (multicast) packet searching
for the DHCP server. When a server replies, the DHCP client sends a message asking for a
lease of an IP address, and the server replies, listing an IPv6 address, prefix length, and
DNS IP addresses. (Note that Stateful DHCPv6 does not supply the default router information,
instead relying on Neighbor Discovery Protocol between the client and local
routers.) The names and formats of the actual DHCP messages have changed quite a bit
from IPv4 to IPv6, so DHCPv4 and DHCPv6 actually differ in detail, but the basic process
remains the same. (The term DHCPv4 refers to the version of DHCP used for IPv4, and the
term DHCPv6 refers to the version of DHCP used for IPv6.)
DHCPv4 servers retain state information about each client, such as the IP address leased
to that client, and the length of time for which the lease is valid. In other words, DHCPv4
tracks the current state of DHCP clients. DHCPv6 servers happen to have two operational
modes: stateful, in which the server does track state information, and stateless, in which
the server does not track any state information. Stateful DHCPv6 servers fill the same role
as the older DHCPv4 servers, whereas stateless DHCPv6 servers fill a different purpose as
one part of the stateless autoconfiguration process. (Stateless DHCP, and its purpose, is
covered in the upcoming section “Finding the DNS IP Addresses Using Stateless DHCP.”)
One difference between DHCPv4 and stateful DHCPv6 is that IPv4 hosts send IP broadcasts
to find DHCP servers, whereas IPv6 hosts send IPv6 multicasts. IPv6 multicast addresses
have a prefix of FF00::/8, meaning that if the first 8 bits of an address are binary
11111111, or FF in hex. The multicast address FF02::1:2 (longhand
FF02:0000:0000:0000:0000:0000:0001:0002) has been reserved in IPv6 to be used by
hosts to send packets to an unknown DHCP server, with the routers working to forward
these packets to the appropriate DHCP server.
Stateless Autoconfiguration
The second of the two options for dynamic IPv6 address assignment uses a built-in IPv6
feature called stateless autoconfiguration as the core tool. Stateless autoconfiguration allows
a host to automatically learn the key pieces of addressing information–prefix, host,
and prefix length–plus the default router IP address and DNS IP addresses. To learn or derive
all these pieces of information, stateless autoconfiguration actually uses the following
functions:
Step 1. IPv6 Neighbor Discovery Protocol (NDP), particularly the router solicitation
and router advertisement messages, to learn the prefix, prefix length, and default
router
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Step 2. Some math to derive the interface ID (host ID) portion of the IPv6 address, using
a format called EUI-64
Step 3. Stateless DHCP to learn the DNS IPv6 addresses
This section examines all three topics in order.
Learning the Prefix/Length and Default Router with NDP Router
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The IPv6 Neighbor Discovery Protocol (NDP) has many functions. One function allows
IPv6 hosts to multicast a message that asks all routers on the link to announce two key
pieces of information: the IPv6 addresses of routers willing to act as a default gateway and
all known IPv6 prefixes on the link. This process uses ICMPv6 messages called a Router
Solicitation (RS) and a Router Advertisement (RA).
For this process to work, before a host sends an RS message on a LAN, some router connected
to that same LAN must already be configured for IPv6. The router must have an
IPv6 address configured, and it must be configured to route IPv6 traffic. At that point, the
router knows it can be useful as a default gateway, and it knows at least one prefix that can
be useful to any clients on the LAN.
For example, Figure 16-7 shows a subset of the internetwork seen in Figures 16-5 and 16-
6, with the same IPv6 addresses and subnets used. Router R1’s Fa0/0 has already been configured
with an IPv6 address (2340:1111:AAAA:1:213:19FF:FE7B:5004/64) and has been
configured to route IPv6 with the ipv6 unicast-routing global command.
In the figure, host PC1, using stateless autoconfig, sends the RS message as an IPv6 multicast
message destined to all IPv6 routers on the local link. The RS asks all routers to respond
to the questions “What IPv6 prefix(s) is used on this subnet?” and “What is the
IPv6 address(s) of any default routers on this subnet?” The figure also shows R1’s response
(RS), listing the prefix (2340:1111:AAAA:1::/64), and with R1’s own IPv6 address as a potential
default router.
Chapter 16 : IP Version 6 Addressing 547
Note: IPv6 allows for multiple prefixes and multiple default routers to be listed in the RA
message; Figure 16-7 just shows one of each for simplicity’s sake. One router’s RA would
include IPv6 addresses and prefixes advertised by other routers on the link as well.
IPv6 does not use broadcasts. In fact, there is no such thing as a subnet broadcast address,
a networkwide broadcast address, or an equivalent of the all-hosts 255.255.255.255 broadcast
IPv4 address. Instead, IPv6 makes use of multicast addresses. By defining a different
multicast IPv6 address for different functions, an IPv6 host that has no need to participate
in a particular function can simply ignore those particular multicasts, reducing the impact
to the host.
For example, the RS message needs to be received and processed only by routers, so the
RS message’s destination IP address is FF02::2, which IPv6 reserves for use only by IPv6
routers. IPv6 defines that routers send RA messages to a multicast address intended for
use by all IPv6 hosts on the link (FF02::1); routers do not forward these messages to other
links. As a result, not only does the host that sent the RS message learn the information,
all other hosts on the link learn the details as well. Table 16-6 summarizes some of the key
details about the RS/RA messages.
Calculating the Interface ID Using EUI-64
Earlier in the chapter, Figure 16-5 shows the format of an IPv6 global unicast address with
the second half of the address called the host ID or interface ID. The value of the interface
ID portion of a global unicast address can be set to any value if no other host in the same
subnet attempts to use the same value.
To automatically create a guaranteed-unique interface ID, IPv6 defines a method to calculate
a 64-bit interface ID derived from that host’s MAC address. Because the burned-in
MAC address should be literally globally unique, the derived interface ID should also be
globally unique as well.
The EUI-64 process takes the 6-byte (48-bit) MAC address and expands it into a 64-bit
value. To do so, IPv6 fills in 2 more bytes into the middle of the MAC address. IPv6 separates
the original MAC address into two 3-byte halves and inserts hex FFFE in between
the halves to form the Interface ID field of the IPv6 address. The conversion also requires
flipping the seventh bit inside the IPv6 address, resulting in a 64-bit number that conforms
to a convention called the EUI-64 format. The process is shown in Figure 16-8.
Although it may seem a bit convoluted, it works. Also, with a little practice, you can look
at an IPv6 address and quickly notice the FFFE late in the address and then easily find the
two halves of the corresponding interface’s MAC address.
Table 16-6 Details of the RS/RA Process
Message RS RA
Multicast destination FF02::2 FF02::1
Meaning of Multicast address All routers on this link All IPv6 nodes on this link
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Flip 7th Bit
(Reading Left to Right)
in First Byte
Subnet Prefix
Site Prefix
48 Bits 16 Bits 64 Bits
Prefix (ISP-assigned) Subnet Interface ID
1st Half of FFFE
MAC
2nd Half of
MAC
EUI-64 Format
Figure 16-8 IPv6 Address Format with Interface ID and EUI-64
For example, the following two lines list a host’s MAC address, and corresponding EUI-64
format Interface ID, assuming the use of an address configuration option that uses the
EUI-64 format:
0034:5678:9ABC
0234:56FF:FE78:9ABC
Note: To change the seventh bit (left-to-right) in the example, hex 00 converts to binary
00000000, change the seventh bit to 1 (00000010), convert back to hex, for hex 02 as the
first two digits.
At this point in the stateless autoconfig process, a host knows its full IPv6 address and
prefix length, plus a local router to use as default gateway. The next section discusses how
to complete the process using stateless DHCP.
Finding the DNS IP Addresses Using Stateless DHCP
Although the DHCP server function for IPv4 does not explicitly use the word “stateful” in
its name, IPv4 DHCP servers keep state information about DHCP clients. The server keeps
a record of the leased IP addresses, and when the lease expires. The server typically releases
the addresses to the same client before the lease expires, and if no response is heard
from a DHCP client in time to renew the lease, the server releases that IP address back into
the pool of usable IP addresses–again keeping that state information. The server also has
configuration of the subnets in uses and a pool of addresses in most subnets from which
the server can assign IP addresses. It also serves other information, such as the default
router IP addresses in each subnet, and the DNS server IP addresses.
The IPv6 stateful DHCP server, as previously discussed in the section “Stateful DHCP for
IPv6”, follows the same general idea. However, for IPv6, this server’s name includes the
word stateful, to contrast it with the stateless DHCP server function in IPv6.
The stateless DHCP server function in IPv6 solves one particular problem: It supplies the
DNS server IPv6 address(es) to clients. Because all hosts typically use the same small
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Chapter 16 : IP Version 6 Addressing 549
number of DNS servers, the stateless DHCP server does not need to keep track of any
state information. An engineer simply configures the stateless DHCP server to know the
IPv6 addresses of the DNS servers, and the servers tells any host or other device that asks,
keeping no record of the process.
Hosts that use stateless autoconfig also use stateless DHCP to learn the DNS server IPv6
addresses.
Table 16-7 summarizes some of the key features of stateful and stateless DHCPv6.
Static IPv6 Address Configuration
Two options exist for static configuration of IPv6 addresses. For one option, you configure
the entire 128-bit IPv6 address, and for the other, you just configure the 64-bit prefix
and tell the device to use an EUI-64 calculation for the interface ID portion of the address.
Both options result in the host or router interface knowing its full 128-bit IPv6 address
and prefix length.
When a host uses either form of static IPv6 address configuration, the host does not need
to statically configure the other key pieces of information (default router and DNS IP addresses).
The host can use the usual NDP process to discover any default routers and
stateless DHCP to discover the DNS IPv6 addresses.
When a router uses static IPv6 address configuration, it may still use stateless DHCP to
learn the DNS IP addresses. The upcoming section “Configuring IPv6 Addresses on Cisco
Routers” shows several examples of this configuration.
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