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This section covers the major types of IPv6 addresses. IPv4 addresses are unicast, multicast, or broadcast. IPv6 maintains each of these address functions, except that the IPv6 address types are defined a little differently. A special "all-nodes" IPv6 multicast address handles the broadcast function. IPv6 also introduces the anycast address type.
Also important to understand are the IPv6 address allocations. Sections of the IPv6 address space are reserved for particular functions, each of which is covered in this section. To provide you with a full understanding of address types, the following sections describe each type.
- Unicast
- Anycast
- Multicast
IPv6 Unicast Address
The IPv6 unicast (one-to-one) address is the logical identifier of a single-host interface. It is similar to IPv4 unicast classful (Class A, Class B, and Class C) addresses. Unicast addresses are divided into global and link-local addresses. A third type, site-local, has been deprecated in RFC 3879. These unicast address types are explained in the following sections.
IPv6 Anycast Address
The IPv6 anycast (one-to-nearest) address identifies a set of devices. An anycast address is allocated from a set of unicast addresses. These destination devices should share common characteristics and are explicitly configured for anycast.
You can use the anycast address to identify a set of routers or servers within an area. When a packet is sent to the anycast address, it is delivered to the nearest device as determined by the routing protocol. An example of the use of anycast addresses is to assign an anycast address to a set of servers—one in North America, and the other in Europe. Users in North America would be routed to the North American server, and those in Europe to the European server.
IPv6 Multicast Address
The IPv6 multicast (one-to-many) address identifies a set of hosts. The packet is delivered to all the hosts identified by that address. This type is similar to IPv4 multicast (Class D) addresses. IPv6 multicast addresses also supersede the broadcast function of IPv4 broadcasts. You use an "all-nodes" multicast address instead.
Some IPv6 multicast addresses are
FF01:0:0:0:0:0:0:1— Indicates all-nodes address for interface-local scope.
FF02:0:0:0:0:0:0:2— All-routers address for link-local.
IPv6 Address Allocations
The leading bits of an IPv6 address can define the IPv6 address type or other reservations. These leading bits are of variable length and are called the format prefix (FP). Table 8-2 shows the allocation of address prefixes. The IPv6 address space was delegated to IANA. You can find current IPv6 allocations at http://www.iana.org/assignments/ipv6-address-space. Many prefixes are still unassigned.
Unspecified Address
An unspecified address is all 0s: 0:0:0:0:0:0:0:0. It signifies that an IPv6 address is not specified for the interface. Unspecified addresses are not forwarded by an IPv6 router.
Loopback Address
The IPv6 loopback address is 0:0:0:0:0:0:0:1. This address is similar to the IPv4 loopback address of 127.0.0.1.
IPv4-Compatible IPv6 Address
IPv4-compatible IPv6 addresses begin with 96 binary 0s (six 16-bit groups) followed by the 32-bit IPv4 address, as in 0:0:0:0:0:0:130.100.50.1 or just ::130.100.50.1.
Global Unicast Addresses
IPv6 global addresses connect to the public network. These unicast addresses are globally unique and routable. This address format is initially defined in RFC 2374. RFC 3587 provides updates to the format.
The original specification defined the address format with a three-layer hierarchy: public topology, site topology, and interface identifier. The public topology consisted of service providers that provided transit services and exchanges of routing information. It used a Top-Level Aggregator (TLA) identifier and a next-level identifier. A site-level aggregator (SLA) was used for site topology. The site topology is local to the company or site and does not provide transit services. The TLA, NLA, and SLA identifiers are deprecated by RFC 3587. RFC 3587 simplifies these identifiers with a global routing prefix and subnet identifier for the network portion of the address.
Figure 8-2 shows the format of the standard IPv6 global unicast address. The global routing prefix is generally 48 bits in length, and the subnet ID is 16 bits. The interface ID is 64 bits in length and uniquely identifies the interface on the link.
Link-Local Addresses
IPv6 link-local addresses are significant only to nodes on a single link. Routers do not forward packets with a link-local source or destination address beyond the local link. Link-local addresses are identified by leading FE8 hexadecimal numbers. Link-local addresses are configured automatically or manually.
As shown in Figure 8-3, the format of the link-local address is an FP of 1111111010, followed by 54 0s and a 64-bit interface identifier (ID). The interface ID is obtained automatically through communication with other nodes in the link. The interface ID is then concatenated with the link-local address prefix of FE80::/64 to obtain the interface link-local address.
Figure 8-3. IPv6 Link-Local Address Format
Site-Local Addresses
Site-local addresses were recently removed from IPv6 specifications. They are included here in case you encounter them in other references. IPv6 site-local addresses were meant to be analogous to IPv4 private addresses (RFC 1918). Site-local addresses were meant to be used within an organization and are not globally unique. Site-local addresses are not routable across a public network such as the Internet.
Multicast Addresses
IPv6 multicast addresses perform the same function as IPv4 multicast addresses. Multicast addresses send packets to all hosts in a group. IPv6 multicast addresses are identified by the leading FF hexadecimal numbers (an FP value of 11111111). One additional function of IPv6 multicast is to provide the IPv4 broadcast equivalent with the all-nodes multicast group.
RFC 2373 specifies the format of IPv6 multicast addresses. As shown in Figure 8-4, the fields of the IPv6 multicast address are the FP, a value of 0xFF, followed by a 4-bit flags field, a 4-bit scope field, and 112 bits for the group identifier (ID).
Figure 8-4. Multicast Address Format
The FLGS (flags) field consists of three leading 0s followed by a T bit: 000T. If T = 0, the address is a well-known multicast address assigned by the global IANA. If T = 1, the address is not a permanently assigned address.
The SCOP (scope) field limits the scope of the multicast group. Table 8-3 shows the assigned scope values.
| SCOP (Binary) | SCOP (Hexadecimal) | Assignment |
|---|---|---|
| 0000 | 0 | Reserved |
| 0001 | 1 | Node-local scope |
| 0010 | 2 | Link-local scope |
| 0011 | 3 | Unassigned |
| 0100 | 4 | Admin-local scope |
| 0101 | 5 | Site-local scope |
| 0110 | 6 | Unassigned |
| 0111 | 7 | Unassigned |
| 1000 | 8 | Organization-local scope |
| 1001 | 9 | Unassigned |
| 1010 | A | Unassigned |
| 1011 | B | Unassigned |
| 1100 | C | Unassigned |
| 1101 | D | Unassigned |
| 1110 | E | Global scope |
| 1111 | F | Reserved |
The group ID identifies the multicast group within the given scope. The group ID is independent of the scope. A group ID of 0:0:0:0:0:0:1 identifies nodes, whereas a group ID of 0:0:0:0:0:0:2 identifies routers. Some well-known multicast addresses appear in Table 8-4 associated with a variety of scope values.
| Multicast Address | Multicast Group |
|---|---|
| FF01::1 | All nodes (node-local) |
| FF02::1 | All nodes (link-local) |
| FF01::2 | All routers (node-local) |
| FF02::2 | All routers (link-local) |
| FF02::5 | Open Shortest Path First version 3 (OSPFv3) |
| FF02::6 | OSPFv3 designated routers |
| FF02::9 | Routing Information Protocol (RIPng) |
| FF02::A | EIGRP routers |
| FF02::B | Mobile agents |
| FF02::C | DHCP servers/relay agents |
| FF02::D | All Protocol Independent Multicast (PIM) routers |
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