Pv6 Address Types
Like IPv4 addresses, several types of IPv6 addresses are required for the various applications of IPv6 as a Layer 3 protocol. In IPv4, the address types are unicast, multicast, and broadcast. IPv6 differs slightly in that broadcast addressing is not used; special multicast addresses take the place of IPv4 broadcast addresses. However, three address types remain in IPv6: unicast, multicast, and anycast. This section of the chapter discusses each one. Table 20-2 summarizes the IPv6 address types.
748 Chapter 20: IP Version 6 Table 20-2 IPv6 Address Types
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Address Type |
Range |
Application |
|
Aggregatable global unicast |
2000::/3 |
Host-to-host communication; same as IPv4 unicast. |
|
Multicast |
FF00::/8 |
One-to-many and many-to-many communication; same as IPv4 multicast. |
|
Anycast |
Same as Unicast |
Application-based, including load balancing, optimizing traffic for a particular service, and redundancy. Relies on routing metrics to determine the best destination for a particular host. |
|
Link-local unicast |
FE80::/10 |
Connected-link communications. |
|
Solicited-node multicast |
FF02::1:FF00:0/104 |
Many of the terms in Table 20-2 are exclusive to IPv6. The following sections examine each of the address types listed in the table.
Unicast
Unicast IPv6 addresses have much the same functionality as unicast IPv4 addresses, but because IPv6's 128-bit address space provides so many more addresses to use, we have much more flexibility in assigning them globally. Because one of the intents for IPv6 addressing in public networks is to allow wide use of globally unique addresses, aggregatable global unicast IPv6 addresses are allocated in a way in which they can be easily summarized to reasonably contain the size of global IPv6 routing tables in service provider networks.
In addition to aggregatable global unicast addresses, several other aspects of IPv6 unicast addressing deserve mention here and follow in the next few sections.
Aggregatable Global Addresses
In current usage, aggregatable global addresses are assigned from the IPv6 addresses that begin with binary 001. This value can be written in prefix notation as 2000::/3, which means "all IPv6 addresses whose first 3 bits are equal to the first 3 bits of hex 2000." In practice, this includes IPv6 addresses that begin with hex 2 or 3. (Note that RFC 3587 later removed the restriction to only allocate aggregatable global unicast addresses from the 2000::/3, but in practice, these addresses are still allocated from this range.) To ensure that IPv6 addresses can be summarized efficiently when advertised toward Internet routers, several global organizations allocate these addresses to service providers and other users. See RFC 3587 and RFC 3177 for more details.
Aggregatable global address prefixes are structured so that they can be strictly summarized and aggregated through a hierarchy consisting of a private network and a series of service providers. Here is how that works, based on RFC 3177, starting after the first 3 bits in the prefix:
The next 45 bits represent the global routing prefix.
The last 16 bits in the prefix, immediately preceding the Interface ID portion of the address, are Site Level Aggregator (SLA), bits. These bits are used by an organization for its own internal addressing hierarchy. This field is also known as the Subnet ID.
■ The last 64 bits make up the interface ID.
Figure 20-1 shows the aggregatable global unicast IPv6 address format. Figure 20-1 IPv6 Address Format
Key Topic
Figure 20-1 shows the aggregatable global unicast IPv6 address format. Figure 20-1 IPv6 Address Format
The interface ID portion of an aggregatable global IPv6 address can be explicitly assigned in Cisco IOS or derived using a number of methods explored later in this chapter in the "IPv6 Address Autoconfiguration" section. These addresses should use an Interface ID in the modified EUI-64 format, discussed later in this chapter. Depending on how these addresses are assigned, however, the Universal/Local bit, which is the 7th bit in the Interface ID field of an IPv6 address, can be set to 0 (locally administered) or 1 (globally unique) to indicate the nature of the Interface ID portion of the address.
Link-Local Addresses
As the term implies, link-local addresses are used on a data link or multiaccess network, such as a serial link or an Ethernet network. Because these addresses are link-local in scope, they are guaranteed to be unique only on that link or multiaccess network. Each interface type, regardless of whether it is serial, PPP, ATM, Frame Relay, Ethernet, or something else, gets a link-local address when IPv6 is enabled on that interface.
Link-local addresses always begin with FE80::/10. The Interface ID portion of the address is derived using the modified EUI-64 format, discussed later in this chapter. The remaining 54 bits of the prefix are always set to 0.
On Ethernet interfaces, the IEEE 802 MAC address is the basis for the Interface ID. For other interface types, routers draw from a pool of virtual MAC addresses to generate the Interface IDs. An example of a fully formed link-local address follows:
FE80::207:85FF:FE80:71B8
As you might gather from the name, link-local addresses are used for communication between hosts that do not need to leave the local segment. By definition, routers do not forward link-local traffic to other segments. As you will see later in this chapter, link-local addresses are used for operations such as routing protocol neighbor communications, which are by their nature link-local.
IPv4-Compatible IPv6 Addresses
Many transition strategies have been developed for IPv4 networks to migrate to IPv6 service and for IPv6 networks to intercommunicate over IPv4 networks. Most of these strategies involve tunneling. Similarly, a mechanism exists for creating IPv6 addresses that are compatible with IPv4. These addresses use 0s in the first 96 bits of the address and one of the two formats for the remaining portion of the address. Take a look at an example, given the IPv4 address 10.10.100.16. The following are valid IPv4-compatible IPv6 addresses that correspond to this IPv4 address (all of these are in hexadecimal, as IPv6 addresses are universally represented):
IPv4-compatible IPv6 addresses are not widely used and do not represent a design best practice, but you should be familiar with their format. See the section "Tunneling," later in this chapter for more detail on IPv4-compatible address usage in the corresponding tunnel type and on the deprecation of this tunneling type in Cisco IOS.
Assigning an IPv6 Unicast Address to a Router Interface
To configure any IPv6 address or other IPv6 feature, you must first globally enable IPv6 on the
Topic router or switch:
Stengel(config)# ipv6 unicast-routing
Next, configure a global unicast address:
Stengel(config-if)# ipv6 address 2001:128:ab2e:1a::1/64 Routers automatically configure a link local IPv6 address on all IPv6-enabled interfaces. However, you can configure the link local address with the following command. (Note the the link-local keyword to designate the address type.)
Stengel(config-if)# ipv6 address fe80::1 link-local ..■■— Unlike IPv4, IPv6 allows you to assign many addresses to an interface. All IPv6 addresses
\ Topic configured on an interface get equal precedence in terms of IP routing behavior. Multicast
Multicast for IPv6 functions much like IPv4 multicast. It allows multiple hosts to become members of (that is, receive traffic sent to) a multicast group without regard to their location or number. A multicast receiver is known as a group member, because it joins the multicast group to receive traffic. Multicast addresses in IPv6 have a specific format, which is covered in the next section.
Because IPv6 has no broadcast addressing concept, multicast takes the place of all functions that would use broadcast in an IPv4 network. For example, the IPv6 DHCP process uses multicast for sending traffic to an unknown host on a local network.
As in IPv4, IPv6 multicast addresses are always destinations; a multicast address cannot be used as a source of any IPv6 traffic.
IPv6 multicast is covered in more detail in the last section of this chapter. IPv6 Multicast Address Format
Multicast addresses in IPv6 always begin with FF as the first octet in the address, or FF00::/8. The second octet specifies the lifetime and scope of the multicast group. Lifetime can be permanent or temporary. Scope can be local to any of the following:
■ Organization
The multicast address format is shown in Figure 20-2. Figure 20-2 IPv6 Multicast Address Format
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16 Bits -*-V |
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All 0s |
8 Bits FF 8 Bits Scope 4 Bits 4 Bits 64 Bits Scope 0001 = Node 0010 = Link 0101 = Site 1000 = Organization 1110 = Global 0000 0001 = Permanent = Temporary Table 20-3 shows several well-known IPv6 multicast group addresses and their functions. Readers' Questions
In an IPv6 network, as in IPv4, there is an all-nodes multicast group (FF02::1), of which all IPv6 hosts are members. All routers must join the all-routers multicast address (FF02::2). In addition, IPv6 multicast uses a solicited-node group that each router must join for all of its unicast and anycast addresses. The format for solicited-node multicast addresses is FF02::1:FF00:0000/104 Note that all but the last 24 bits of the address are specified by the /104 prefix. Solicited-node addresses are built from this prefix concatenated with the low-order 24 bits (128 - 104 = 24) of the corresponding unicast or anycast address. For example, a unicast address of 2001:1AB:2003:1::CBAC:DF01 has a corresponding solicited-node multicast address of FF02::1:FFAC:DF01 Solicited-node addresses are used in the Neighbor Discovery (ND) process, covered later in this chapter. Multicast in IPv6 relies on a number of protocols with which you are already familiar, including PIM. Multicast Listener Discovery is another key part of IPv6 multicast. These topics and other related multicast subjects are covered later in this chapter in the "IPv6 Multicast" section. AnycastIn some applications, particularly server farms or provider environments, it may be desirable to pool a number of servers to provide redundancy, load balancing, or both. Several protocols can provide this functionality in IPv4 networks. IPv6 has built-in support for this application in the form of anycast addressing. Anycast addresses can be assigned to any number of hosts that provide the same service; when other hosts access this service, the specific server they hit is determined by the unicast routing metrics on the path to that particular group of servers. This provides geographic differentiation, enhanced availability, and load balancing for the service. /— Anycast addresses are drawn from the IPv6 unicast address pool and, therefore, are not [ Topic distinguishable from unicast addresses. RFC 2526 recommends a range of addresses for use by anycast applications. Once an address is assigned to more than one host, it becomes an anycast address by definition. Because anycast addresses cannot be used to source traffic, however, a router must know if one of its interface IPv6 addresses is an anycast address. Therefore, Cisco IOS Software requires the anycast keyword to be applied when an anycast address is configured, as in this example: Mariano(config-if)# ipv6 address 3001:fffe::104/64 anycast All IPv6 routers additionally must support the subnet router anycast address. This anycast address is a prefix followed by all 0s in the interface ID portion of the address. Hosts can use a subnet router anycast address to reach a particular router on the link identified by the prefix given in the subnet router anycast address. The Unspecified Address ..■■— One additional type of IPv6 address deserves mention in this section, as it is used for a number of Topic functions in IPv6 communications. This address, which is used for some types of requests covered later in this chapter, is represented simply by ::. The unspecified address is always a source address used by an interface that has not yet learned its unicast address. The unspecified address cannot be assigned to an interface, and it cannot be used as a destination address. Continue reading here: Unicast Reverse Path Forwarding Was this article helpful? |
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