Understanding IPv6

Author Joseph Davies Pages 544 Disk 1 Companion CD(s) Level Int/Adv Published 11/13/2002 ISBN 9780735612457 Price $29.99 To see this book's discounted price, select a reseller below.	More Information About the Book Table of Contents Sample Chapter Index Related Series Related Books About the Author Support: Book & CD Rate this book

Chapter 3: IPv6 Addressing

• The IPv6 Address Space
• Current Allocation
• IPv6 Address Syntax
• Compressing Zeros
• IPv6 Prefixes
• Types of IPv6 Addresses
• Unicast IPv6 Addresses
• Aggregatable Global Unicast Addresses
• Topologies Within Global Addresses
• Local-Use Unicast Addresses
• Special IPv6 Addresses
• Compatibility Addresses
• NSAP Addresses
• Multicast IPv6 Addresses
• Recommended Multicast IPv6 Addresses
• Solicited-Node Address
• Anycast IPv6 Addresses
• Subnet-Router Anycast Address
• IPv6 Addresses for a Host
• IPv6 Addresses for a Router
• Subnetting the IPv6 Address Space
• Subnetting for NLA IDs
• Subnetting for SLA IDs/Subnet IDs
• IPv6 Interface Identifiers
• EUI-64 Address-based Interface Identifiers
• Temporary Address Interface Identifiers
• Mapping IPv6 Multicast Addresses to Ethernet Addresses
• IPv4 Addresses and IPv6 Equivalents
• References
• Testing for Understanding

Chapter 3  IPv6 Addressing

• Describe the IPv6 address space and state why the address length of 128 bits was chosen.
• Describe IPv6 address syntax, including zero suppression and compression and prefixes.
• Enumerate and describe the function of the different types of unicast IPv6 addresses.
• Describe the format of multicast IPv6 addresses.
• Describe the function of anycast IPv6 addresses.
• Describe how IPv6 interface identifiers are derived.
• List and compare the different addressing concepts between IPv4 addresses and IPv6 addresses.

The IPv6 Address Space

In the late 1970s, when the IPv4 address space was designed, it was unimaginable that it could ever be exhausted. However, due to changes in technology and an allocation practice that did not anticipate the recent explosion of hosts on the Internet, the IPv4 address space was consumed to the point that by 1992, it was clear a replacement would be necessary.

With IPv6, it is even harder to conceive that the IPv6 address space will ever be consumed. To help put this number in perspective, a 128-bit address space provides 665,570,793,348,866,943,898,599 (6.65 x 10²³) addresses for every square meter of the Earth's surface.

It is important to remember that the decision to make the IPv6 address 128 bits in length was not so that every square meter of the Earth could have 6.65 x 10²³ addresses. Rather, the relatively large size of the IPv6 address is designed to be divided into hierarchical routing domains that reflect the topology of the modern-day Internet. The use of 128 bits allows for multiple levels of hierarchy and flexibility in designing hierarchical addressing and routing that is currently lacking on the IPv4-based Internet.

It is easy to get lost in the vastness of the IPv6 address space. As we will discover, the unthinkably large 128-bit IPv6 address that is assigned to an interface on a typical IPv6 host is composed of a 64-bit subnet identifier and a 64-bit interface identifier (a 50-50 split between subnet space and interface space). The 64 bits of subnet identifier leave enough addressing room to satisfy the addressing requirements of three levels of Internet service providers (ISPs) between your organization and the backbone of the Internet and the addressing needs of your organization. The 64 bits of interface identifier accommodate the mapping of current and future link-layer media access control (MAC) addresses.

Current Allocation

Table 3-1 lists the allocation of the IPv6 address space by FPs as defined in RFC 2373.

Table 3-1. Current Allocation of the IPv6 Address Space

Allocation Space	Format Prefix (FP)	Fraction of the Address
Reserved	0000 0000	1/256
Unassigned	0000 0001	1/256
Reserved for Network Service Access Point (NSAP) allocation	0000 001	1/128
Unassigned	0000 010	1/128
Unassigned	0000 011	1/128
Unassigned	0000 1	1/32
Unassigned	0001	1/16
Aggregatable global unicast addresses	001	1/8
Unassigned	010	1/8
Unassigned	011	1/8
Unassigned	100	1/8
Unassigned	101	1/8
Unassigned	110	1/8
Unassigned	1110	1/16
Unassigned	1111 0	1/32
Unassigned	1111 10	1/64
Unassigned	1111 110	1/128
Unassigned	1111 1110 0	1/512
Link-local unicast addresses	1111 1110 10	1/1024
Site-local unicast addresses	1111 1110 11	1/1024
Multicast addresses	1111 1111	1/256

The current set of unicast addresses that can be used with IPv6 nodes consists of aggregatable global unicast addresses, link-local unicast addresses, and site-local unicast addresses. These addresses represent only 12.7 percent of the entire IPv6 address space.

IPv6 Address Syntax

The following is an IPv6 address in binary form:

0010000111011010000000001101001100000000000000000010111100111011

0000001010101010000000001111111111111110001010001001110001011010

The 128-bit address is divided along 16-bit boundaries:

0010000111011010  0000000011010011  0000000000000000  0010111100111011

0000001010101010  0000000011111111  1111111000101000  1001110001011010

Each 16-bit block is converted to hexadecimal and delimited with colons. The result is:

21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A

IPv6 address representation is further simplified by suppressing the leading zeros within each 16-bit block. However, each block must have at least a single digit. With leading zero suppression, the result is:

21DA:D3:0:2F3B:2AA:FF:FE28:9C5A

Table 3-2 lists the conversion between binary, hexadecimal, and decimal numbers.

Table 3-2. Converting Between Binary, Hexadecimal, and Decimal Numbers

Binary	Hexadecimal	Decimal
0000	0	0
0001	1	1
0010	2	2
0011	3	3
0100	4	4
0101	5	5
0110	6	6
0111	7	7
1000	8	8
1001	9	9
1010	A	10
1011	B	11
1100	C	12
1101	D	13
1110	E	14
1111	F	15

Compressing Zeros

For example, the link-local address of FE80:0:0:0:2AA:FF:FE9A:4CA2 can be compressed to FE80::2AA:FF:FE9A:4CA2. The multicast address FF02:0:0:0:0:0:0:2 can be compressed to FF02::2.

IPv6 Prefixes

For example, 21DA:D3::/48 is a route prefix and 21DA:D3:0:2F3B::/64 is a subnet prefix. As described earlier in this chapter, the 64-bit prefix is used for individual subnets to which nodes are attached. All subnets have a 64-bit prefix. Any prefix that is less than 64 bits is a route or address range that is summarizing a portion of the IPv6 address space.

An IPv6 prefix is relevant only for routes or address ranges, not for individual unicast addresses. In IPv4, it is common to express an IPv4 address with its prefix length. For example, 192.168.29.7/24 (equivalent to 192.168.29.7 with the subnet mask 255.255.255.0) denotes the IPv4 address 192.168.29.7 with a 24-bit subnet mask. Because IPv4 addresses are no longer class-based, you cannot assume the class-based subnet mask based on the value of the leading octet. The prefix length is included so that you can determine which bits identify the subnet and which bits identify the host on the subnet. Because the number of bits used to identify the subnet in IPv4 is variable, the prefix length is needed to separate the subnet ID from the host ID.

In IPv6, however, there is no notion of a variable length subnet identifier. At the individual IPv6 subnet level for currently defined unicast IPv6 addresses, the number of bits used to identify the subnet is always 64 and the number of bits used to identify the host on the subnet is always 64. Therefore, while unicast IPv6 addresses written with their prefix lengths are permitted in RFC 2373, in practice their prefix lengths are always 64 and therefore do not need to be expressed. For example, there is no need to express the IPv6 unicast address FEC0::2AC4: 2AA:FF:FE9A:82D4 as FEC0::2AC4:2AA:FF:FE9A:82D4/64. Due to the 50-50 split of subnet and interface identifiers, the unicast IPv6 address FEC0::2AC4:2AA: FF:FE9A:82D4 implies that the subnet identifier is FEC0:0:0:2AC4::/64.

Types of IPv6 Addresses

1. Unicast

A unicast address identifies a single interface within the scope of the type of address. The scope of an address is the region of the IPv6 network over which the address is unique. With the appropriate unicast routing topology, packets addressed to a unicast address are delivered to a single interface. To accommodate load-balancing systems, RFC 2373 allows for multiple interfaces to use the same address as long as they appear as a single interface to the IPv6 implementation on the host.

2. Multicast

A multicast address identifies zero or more interfaces. With the appropriate multicast routing topology, packets addressed to a multicast address are delivered to all interfaces identified by the address.

3. Anycast

An anycast address identifies multiple interfaces. With the appropriate unicast routing topology, packets addressed to an anycast address are delivered to a single interface—the nearest interface that is identified by the address. The nearest interface is defined as being the closest in terms of routing distance. A multicast address is used for one-to-many communication, with delivery to multiple interfaces. An anycast address is used for one-to-one-of-many communication, with delivery to a single interface.

In all cases, IPv6 addresses identify interfaces, not nodes. A node is identified by any unicast address assigned to any one of its interfaces.

Unicast IPv6 Addresses

• Aggregatable global unicast addresses
• Link-local addresses
• Site-local addresses
• Special addresses
• Compatibility addresses
• NSAP addresses

Aggregatable Global Unicast Addresses

As the name implies, aggregatable global unicast addresses are designed to be aggregated or summarized to produce an efficient routing infrastructure. Unlike the current IPv4-based Internet, which is a mixture of both flat and hierarchical routing, the IPv6-based Internet has been designed from its foundation to support efficient, hierarchical addressing and routing. The scope of a global address is the entire IPv6 Internet.

Figure 3-1 shows the structure of an aggregatable global unicast address.

Click to view graphic

Figure 3-1. The structure of an aggregatable global unicast address

The fields in the aggregatable global unicast address are:

TLA ID — Top-Level Aggregation Identifier. The size of this field is 13 bits. The TLA ID identifies the highest level in the routing hierarchy. TLA IDs are administered by the Internet Assigned Numbers Authority (IANA) and allocated to local Internet registries that, in turn, allocate individual TLA IDs to large, long-haul ISPs. A 13-bit field allows up to 8,192 different TLA IDs. Routers in the highest level of the IPv6 Internet routing hierarchy (called default-free routers) do not have a default route—only routes with 16-bit prefixes corresponding to the allocated TLA IDs and additional entries for routes based on the TLA ID assigned to the routing region where the router is located.

Res — Bits that are reserved for future use in expanding the size of either the TLA ID or the NLA ID (defined next). The size of this field is 8 bits.

NLA ID — Next-Level Aggregation Identifier. The size of this field is 24 bits. The NLA ID allows an ISP to create multiple levels of addressing hierarchy within its network to both organize addressing and routing for downstream ISPs and identify organization sites. The structure of the ISP's network is not visible to the default-free routers. The combination of the 001 FP, the TLA ID, the Res field, and the NLA ID form a 48-bit prefix that is assigned to an organization's site that is connecting to the IPv6 portion of the Internet. A site is an organization network or portion of an organization's network that has a defined geographical location (such as an office, an office complex, or a campus).

SLA ID — Site-Level Aggregation Identifier. The SLA ID is used by an individual organization to identify subnets within its site. The size of this field is 16 bits. The organization can use these 16 bits within its site to create 65,536 subnets or create multiple levels of addressing hierarchy and an efficient routing infrastructure. With 16 bits of subnetting flexibility, an aggregatable global unicast prefix assigned to an organization is equivalent to that organization being allocated an IPv4 Class A network ID (assuming that the last octet is used for identifying nodes on subnets). The structure of the organization's network is not visible to the ISP.

Interface ID — Indicates the interface on a specific subnet. The size of this field is 64 bits. The interface ID in IPv6 is equivalent to the node ID or host ID in IPv4.

Topologies Within Global Addresses

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Figure 3-2. The topological structure of the global address

The public topology is the collection of larger and smaller ISPs that provide access to the IPv6 Internet. The site topology is the collection of subnets within an organization's site. The interface identifier specifies a unique interface on a subnet within an organization's site.

Local-Use Unicast Addresses

1. Link-local addresses are used between on-link neighbors and for Neighbor Discovery processes.
2. Site-local addresses are used between nodes communicating with other nodes in the same organization.

Link-Local Addresses

Link-local addresses, identified by the FP of 1111 1110 10, are used by nodes when communicating with neighboring nodes on the same link. For example, on a single link IPv6 network with no router, link-local addresses are used to communicate between hosts on the link. Link-local addresses are equivalent to Automatic Private IP Addressing (APIPA) IPv4 addresses autoconfigured on Microsoft Windows .NET Server 2003 family, Windows XP, Windows 2000, Windows Millennium Edition, and Windows 98 computers using the 169.254.0.0/16 prefix. The scope of a link-local address is the local link.

Figure 3-3 shows the structure of the link-local address.

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Figure 3-3. The structure of the link-local address

A link-local address is required for Neighbor Discovery processes and is always automatically configured, even in the absence of all other unicast addresses. For more information about the address autoconfiguration process for link-local addresses, see Chapter 8, "Address Autoconfiguration."

Link-local addresses always begin with FE80. With the 64-bit interface identifier, the prefix for link-local addresses is always FE80::/64. An IPv6 router never forwards link-local traffic beyond the link.

Site-Local Addresses

Site-local addresses, identified by the FP of 1111 1110 11, are equivalent to the IPv4 private address space (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16). For example, private intranets that do not have a direct, routed connection to the IPv6 Internet can use site-local addresses without conflicting with global addresses. Site-local addresses are not reachable from other sites, and routers must not forward site-local traffic outside the site. Site-local addresses can be used in addition to global addresses. The scope of a site-local address is the site.

Figure 3-4 shows the structure of the site-local address.

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Figure 3-4. The structure of the site-local address

Unlike link-local addresses, site-local addresses are not automatically configured and must be assigned either through stateless or stateful address autoconfiguration. For more information, see Chapter 8, "Address Autoconfiguration."

The first 48 bits are always fixed for site-local addresses, beginning with FEC0::/48. After the 48 fixed bits is a 16-bit subnet identifier (Subnet ID field) that provides 16 bits with which you can create subnets within your organization. With 16 bits, you can have up to 65,536 subnets in a flat subnet structure, or you can divide the high-order bits of the Subnet ID field to create a hierarchical and aggregatable routing infrastructure. After the Subnet ID field is a 64-bit Interface ID field that identifies a specific interface on a subnet.

The global address and site-local address share the same structure beyond the first 48 bits of the address. In global addresses, the SLA ID field identifies the subnet within an organization. For site-local addresses, the Subnet ID field performs the same function. Because of this, you can create a subnetted routing infrastructure that is used for both site-local and global addresses.

For example, a specific subnet of your organization can be assigned the global prefix 3FFE:FFFF:4D1C:221A::/64 and the site-local prefix FEC0:0:0: 221A::/64 where the subnet is effectively identified by the SLA ID/Subnet ID value of 221A. While the subnet identifier is the same for both prefixes, routes for both prefixes must still be propagated throughout the routing infrastructure so that addresses based on both prefixes are reachable.

Special IPv6 Addresses

• Unspecified address

The unspecified address (0:0:0:0:0:0:0:0 or ::) is used only to indicate the absence of an address. It is equivalent to the IPv4 unspecified address of 0.0.0.0. The unspecified address is typically used as a source address when a unique address has not yet been determined. The unspecified address is never assigned to an interface or used as a destination address.

• Loopback address

The loopback address (0:0:0:0:0:0:0:1 or ::1) is used to identify a loopback interface, enabling a node to send packets to itself. It is equivalent to the IPv4 loopback address of 127.0.0.1. Packets addressed to the loopback address must never be sent on a link or forwarded by an IPv6 router.

Compatibility Addresses

• IPv4-compatible address

The IPv4-compatible address, 0:0:0:0:0:0:w.x.y.z or ::w.x.y.z (where w.x.y.z is the dotted decimal representation of a public IPv4 address), is used by IPv6/IPv4 nodes that are communicating with IPv6 over an IPv4 infrastructure that uses public IPv4 addresses, such as the Internet.

• IPv4-mapped address

The IPv4-mapped address, 0:0:0:0:0:FFFF:w.x.y.z or ::FFFF: w.x.y.z, is used to represent an IPv4-only node to an IPv6 node. Windows .NET Server 2003 family and Windows XP IPv6 do not support the use of IPv4-mapped addresses.

• 6over4 address

An address of the type [64-bit prefix]:0:0:WWXX:YYZZ, where WWXX: YYZZ is the colon hexadecimal representation of w.x.y.z (a public or private IPv4 address), is used to represent a host for the tunneling mechanism known as 6over4.

• 6to4 address

An address of the type 2002:WWXX:YYZZ:[SLA ID]:[Interface ID], where WWXX:YYZZ is the colon hexadecimal representation of w.x.y.z (a public IPv4 address), is used to represent a node for the tunneling mechanism known as 6to4.

• ISATAP address

An address of the type [64-bit prefix]:0:5EFE:w.x.y.z, where w.x.y.z is a public or private IPv4 address, is used to represent a node for the address assignment mechanism known as Intra-Site Automatic Tunnel Addressing Protocol (ISATAP).

For more information about IPv6 compatibility addresses, see Chapter 11, "Coexistence and Migration."

NSAP Addresses

Click to view graphic

Figure 3-5. The structure of NSAP addresses for IPv6

Multicast IPv6 Addresses

IPv6 multicast addresses have the FP of 1111 1111. Therefore, an IPv6 multicast address always begins with FF. Multicast addresses cannot be used as source addresses or as intermediate destinations in a Routing header. Beyond the FP, multicast addresses include additional structure to identify flags, their scope, and the multicast group. Figure 3-6 shows the structure of the IPv6 multicast address.

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Figure 3-6. The structure of the IPv6 multicast address

The fields in the multicast address are:

Flags — Indicates flags set on the multicast address. The size of this field is 4 bits. As of RFC 2373, the only flag defined is the Transient (T) flag, which uses the low-order bit of the Flags field. When set to 0, the T flag indicates that the multicast address is a permanently assigned (well-known) multicast address allocated by IANA. When set to 1, the T flag indicates that the multicast address is a transient (non-permanently-assigned) multicast address.

Scope — Indicates the scope of the IPv6 network for which the multicast traffic is intended to be delivered. The size of this field is 4 bits. In addition to information provided by multicast routing protocols, routers use the multicast scope to determine whether multicast traffic can be forwarded.

Table 3-3 lists the values for the Scope field assigned in RFC 2373.

Table 3-3. Defined Values for the Scope Field

Scope Field Value	Scope
0	Reserved
1	Node-local scope
2	Link-local scope
5	Site-local scope
8	Organization-local scope
E	Global scope
F	Reserved

For example, traffic with the multicast address of FF02::2 has a link-local scope. An IPv6 router never forwards this traffic beyond the local link.

Group ID - Identifies the multicast group and is unique within the scope. The size of this field is 112 bits. Permanently assigned group IDs are in-dependent of the scope. Transient group IDs are relevant only to a specific scope. Multicast addresses from FF01:: through FF0F:: are reserved, well-known addresses.

To identify all nodes for the node-local and link-local scopes, the following addresses are defined:

• FF01::1 (node-local scope all-nodes multicast address)
• FF02::1 (link-local scope all-nodes multicast address)

To identify all routers for the node-local, link-local, and site-local scopes, the following addresses are defined:

• FF01::2 (node-local scope all-routers multicast address)
• FF02::2 (link-local scope all-routers multicast address)
• FF05::2 (site-local scope all-routers multicast address)

For the current list of permanently assigned IPv6 multicast addresses, see http://www.iana.org/assignments/ipv6-multicast-addresses.

IPv6 multicast addresses replace all forms of IPv4 broadcast addresses. The IPv4 network broadcast (in which all host bits are set to 1 in a classful environment), subnet broadcast (in which all host bits are set to 1 in a non-classful environment), and limited broadcast (255.255.255.255) addresses are replaced by the link-local scope all-nodes multicast address (FF02:01) in IPv6.

Recommended Multicast IPv6 Addresses

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Figure 3-7. The structure of the recommended IPv6 multicast address

Solicited-Node Address

For example, Node A is assigned the link-local address of FE80::2AA:FF: FE28:9C5A and is also listening on the corresponding solicited-node multicast address of FF02::1:FF28:9C5A. (An underline is used to highlight the correspondence of the last six hexadecimal digits.) Node B on the local link must resolve Node A's link-local address FE80::2AA:FF:FE28:9C5A to its corresponding link-layer address. Node B sends a Neighbor Solicitation message to the solicited-node multicast address of FF02::1:FF28:9C5A. Because Node A is listening on this multicast address, it processes the Neighbor Solicitation message and sends a unicast Neighbor Advertisement message in reply.

The result of using the solicited-node multicast address is that link-layer address resolutions, a common occurrence on a link, are not using a mechanism that disturbs all network nodes. By using the solicited-node address, very few nodes are disturbed during address resolution. In practice, due to the relationship between the link-layer MAC address, the IPv6 interface ID, and the solicited-node address, the solicited-node address acts as a pseudo-unicast address for very efficient address resolution. For more information, see "IPv6 Interface Identifiers" in this chapter.

Anycast IPv6 Addresses

For example, for the anycast address 3FFE:2900:D005:6187:2AA:FF:FE89: 6B9A, host routes for this address are propagated within the routing infrastructure of the organization assigned the 48-bit prefix 3FFE:2900:D005::/48. Because a node assigned this anycast address can be placed anywhere on the organization's intranet, source routes for all nodes assigned this anycast address are needed in the routing tables of all routers within the organization. Outside the organization, this anycast address is summarized by the 3FFE:2900:D005::/48 prefix that is assigned to the organization. Therefore, the host routes needed to deliver IPv6 packets to the nearest anycast group member within an organization's intranet are not needed in the routing infrastructure of the IPv6 Internet.

As of RFC 2373, anycast addresses are used only as destination addresses and are assigned only to routers. Anycast addresses are assigned out of the unicast address space and the scope of an anycast address is the scope of the type of unicast address from which the anycast address is assigned. It is not possible to determine if a given destination unicast address is also an anycast address. The only nodes that have this awareness are the routers that use host routes to forward the anycast traffic to the nearest anycast group member and the anycast group members themselves.

Subnet-Router Anycast Address

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Figure 3-8. The structure of the Subnet-Router anycast address

All router interfaces attached to a subnet are assigned the Subnet-Router anycast address for that subnet. The Subnet-Router anycast address is used to communicate with the nearest router connected to a specified subnet.

IPv6 Addresses for a Host

• A link-local address for each interface
• Additional unicast addresses for each interface (which could be a site-local address and one or multiple global addresses)
• The loopback address (::1) for the loopback interface

Typical IPv6 hosts are always logically multihomed because they always have at least two addresses with which they can receive packets—a link-local address for local link traffic and a routable site-local or global address.

Additionally, each interface on an IPv6 host is listening for traffic on the following multicast addresses:

• The node-local scope all-nodes multicast address (FF01::1)
• The link-local scope all-nodes multicast address (FF02::1)
• The solicited-node address for each unicast address
• The multicast addresses of joined groups

Last Updated: October 28, 2002

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