The Different IPv6 Address Types

IPv4 supports four different classes of addresses, which are Anycast, Broadcast, Multicast, and Unicast. While the term Anycast has not been used in previous modules in this guide, it is important to remember that Anycast addresses are not special types of addresses. Instead, an Anycast address is simply an IP address that is assigned to multiple interfaces. Common examples of technologies that use Anycast addressing include IP Multicast implementations and 6to4 relay implementation.

NOTE: 6to4 is a transition mechanism for migrating from IPv4 to IPv6. For the CCNA exam, you only need to know that it exists.

With Anycast addressing, devices use the common address that is closest to them based on the routing protocol metric. The next closest address is then used in the event that the primary address is no longer reachable. This concept is illustrated in Figure 7.5 below:

 

Figure 7.5 – Understanding Anycast Addressing

 

Referencing Figure 7.5, both R1 and R2 have a Loopback 254 interface that is configured using a common address: 15.1.1.254/32. This prefix is then advertised dynamically via EIGRP. By default, both R1 and R2 will prefer the 15.1.1.254/32 prefix via their respective Loopback interfaces, as that is a directly connected subnet. Therefore, the common address used will never result in a conflict on either router.

Assuming normal EIGRP metric calculation, R3 and R5 will prefer the Anycast address advertised by R1 due to the lower IGP metric. Similarly, R4 and R6 will prefer the Anycast address advertised by R2 due to the lower IGP metric. In the event that either R1 or R2 fails, the remaining routers in the network will use the Anycast address advertised by the remaining router. When using Anycast addressing, organisations can use a Unicast address either in the RFC 1918 address space or within their public block.

NOTE: You are not expected to implement any Anycast addressing or solutions in the current CCNA exam. However, it is important to be familiar with the concept. It will make more sense after you have reviewed the routing chapters.

At this level, IPv4 Broadcast, Multicast, and Unicast addresses require no further explanation and will not be described in any additional detail in this module or in the remainder of this guide. While IPv4 supports these four different types of addresses, IPv6 does away with the Broadcast addresses and instead supports only the following types of addresses:

• Link-Local addresses
• Site-Local addresses
• Aggregate Global Unicast addresses
• Multicast addresses
• Anycast addresses
• Loopback addresses
• Unspecified addresses

Link-Local Addresses

IPv6 Link-Local addresses can be used only on the local link (i.e., a shared segment between devices), and are automatically assigned to each interface when IPv6 is enabled on that interface. These addresses are assigned from the Link-Local prefix FE80::/10. Keep in mind that FE80::/10 is the equivalent of FE80:0:0:0:0:0:0:0/10, which can also be represented as FE80:0000:0000:0000:0000:0000:0000:0000/10. To complete the address, bits 11 through 64 are set to 0 and the interface Extended Unique Identifier 64 (EUI-64) is appended to the Link-Local address as the low-order 64 bits. The EUI-64 is comprised of the 24-bit manufacturer ID assigned by the IEEE and the 40-bit value assigned by that manufacturer to its products. EUI-64 addressing is described in greater detail later in this module. The format for a Link-Local address is illustrated in Figure 7.6 below:

 

Figure 7.6 – IPv6 Link-Local Addressing

 

Link-local addresses are unique in that they do not change once assigned to an interface. This means that if an interface is assigned a public IPv6 address (e.g., 2001:1000::1/64) and the public IPv6 prefix was changed (i.e., 2001:2000::1/64), the Link-Local address would not change. This allows the host or router to remain reachable by its neighbour, while IPv6 global Internet addresses change. IPv6 routers should not forward packets that have Link-Local source or destination addresses to other IPv6 routers.

 

Site-Local Addresses

Site-Local addresses are Unicast addresses that are used only within a site. Unlike Link-Local addresses, Site-Local addresses must be configured manually on network devices. These addresses are the IPv6 equivalent of the private IPv4 address space defined in RFC 1918 and can be used by organisations that do not have globally routable IPv6 address space. These addresses are not routable on the IPv6 Internet.

While it is possible to perform NAT for IPv6, it is not recommended; hence, the reason for the much larger IPv6 addresses. Site-Local addresses are comprised of the FEC0::/10 prefix, a 54-bit subnet ID, and an interface identifier in the EUI-64 format used by Link-Local addresses. While the 54 bits in a Link-Local address are set to a value of 0, the same 54 bits in Site-Local addresses are used to create different IPv6 prefixes (up to 254). The format for the Site-Local address is illustrated in Figure 7.7 below:

 

Figure 7.7 –  IPv6 Site-Local Addressing

 

While IPv6 Site-Local addresses are described in this section and are still supported in Cisco IOS software, it is important to know that these addresses are deprecated by RFC 3879 (Deprecating Site Local Addresses). Moreover, RFC 4193 (Unique Local IPv6 Unicast Addresses) describes Unique-Local addresses (ULAs), which serve the same function as Site-Local addresses but they are not routable on the IPv6 global Internet, only within a site.

Unique-Local addresses are assigned from the FC00::/7 IPv6 address block, which is then further divided into two /8 address groups referred to as the assigned and random groups. These two groups are the FC00::/8 and the FD00::/8 IPv6 address blocks. The FC00::/8 block is managed by an allocation authority for /48s in use, while the FD00::/8 block is formed by appending a randomly generated 40-bit string to derive a valid /48 block.

 

Aggregate Global Unicast Addresses

Aggregate Global Unicast addresses are the IPv6 addresses used for generic IPv6 traffic, as well as for the IPv6 Internet. These are similar to the public addresses used in IPv4. From a network addressing point of view, each IPv6 Global Unicast address is comprised of three main sections: the prefix received from the provider (48 bits in length), the site prefix (16 bits in length), and the host portion (64 bits in length). This makes up the 128-bit address used in IPv6.

As we learned earlier in this module, the provider-assigned prefix is assigned to an organisation by an IPv6 provider. By default, these prefixes use /48 prefix lengths. In addition, these prefixes are assigned from the IPv6 address spaces (i.e., the /32 prefix lengths) that are owned by the provider. Each provider will own its own IPv6 address space, and the IPv6 prefix assigned by one provider cannot be used on the network of another provider.

Within a site, administrators can then subnet the provider-assigned 48-bit prefix into 64-bit site prefixes by using bits 49 through 64 for subnetting, allowing for 65,535 different subnets for use within their network. The host portion of an IPv6 address represents the network device or host on the IPv6 subnet. This is represented by the low-order 64 bits of the IPv6 address.

Aggregate Global Unicast addresses for IPv6 are assigned by the Internet Assigned Numbers Authority (IANA) and fall within the IPv6 prefix 2000::/3. This allows for a range of Aggregate Global Unicast addresses from 2000 to 3FFF, as illustrated in Table 7.4 below:

 

Table 7.4 – IPv6 Aggregate Global Unicast Addresses

 

From the 2000::/3 IPv6 block, only three subnets have been allocated for use at the time this module was written. These allocations are illustrated in Table 7.5 below:

 

Table 7.5 –  Assigned IPv6 Aggregate Global Unicast Addresses

 

NOTE: The 6to4 transition addresses and the 6bone prefix are described later in this guide.

Within the range of IPv6 Global Aggregate Unicast addresses, a special experimental range is reserved called ORCHID (an acronym for Overlay Routable Cryptographic Hash Identifiers defined in RFC 4843). ORCHID addresses are non-routed IPv6 addresses used for cryptographic hash identifiers. These addresses use the IPv6 prefix 2001:10::/28. Going into detail on ORCHID addresses is beyond the scope of the current CCNA exam requirements and will not be included in this module or in the remainder of this guide.

Multicast Addresses

The Multicast addresses used in IPv6 are derived from the FF00::/8 IPv6 prefix. In IPv6, Multicast operates in a different manner than that of Multicast in IPv4. IP Multicast is used extensively in IPv6 and replaces IPv4 protocols, such as the Address Resolution Protocol (ARP). In addition, Multicast is used in IPv6 for prefix advertisements and renumbering, as well as for Duplicate Address Detection (DAD). These concepts are all described later in this module.

Multicast packets in IPv6 do not use the TTL value to restrict such packets to the local network segment. Instead, the scoping is defined within the Multicast address itself via the use of the Scope field. IPv6 nodes on a network segment listen to Multicast and may even send Multicast packets to exchange information. This allows all nodes on an IPv6 segment to know about all the other neighbours on that same segment. The format for Multicast addresses used in IPv6 networks is illustrated in Figure 7.8 below:

Figure 7.8 – IPv6 Multicast Addressing

As illustrated in Figure 7.8, the format of the IPv6 Multicast address is slightly different from the formats of the other IPv6 addresses you have learned about up until this point. The first 8 bits of the IPv6 Multicast address represent the Multicast prefix FF::/8. The Flag field in the IPv6 Multicast address is used to indicate the type of Multicast address, either permanent or temporary.

Permanent IPv6 Multicast addresses are assigned by IANA, while temporary IPv6 Multicast addresses can be used in pre-deployment Multicast testing. The Flag field may contain one of the two possible values illustrated in Table 7.6 below:

 

Table 7.6 – IPv6 Permanent and Temporary Multicast Addresses

 

The next 4 bits in the Multicast address represent the scope. In IPv6 Multicasting, this field is a mandatory field that restricts Multicast packets from being sent to other areas in the network. This field essentially provides the same function as the TTL field that is used in IPv4. However, with IPv6, there are several types of scopes, which are listed in Table 7.7 below:

 

Table 7.7 – IPv6 Multicast Address Scopes

 

Within the IPv6 Multicast prefix, certain addresses are reserved. These reserved addresses are referred to as Multicast Assigned addresses, which are presented in Table 7.8 below:

 

Table 7.8 – IPv6 Reserved Multicast Addresses

 

In addition to these addresses, a Solicited-Node Multicast address is enabled automatically for each Unicast and Anycast address configured on a router interface or network host. This address has a Link-Local scope, which means that it will never traverse farther than the local network segment.  Solicited-Node Multicast addresses are used for the following two reasons: the replacement of IPv4 ARP and DAD.

Because IPv6 does not use ARP, Solicited-Node Multicast addresses are used by network hosts and routers to learn the Data Link addresses of neighbouring devices. This allows for the conversion and sending of IPv6 packets to IPv6 hosts and routers as frames. DAD is part of the IPv6 Neighbor Discovery Protocol (NDP), which will be described in detail later in this module. DAD simply allows a device to validate whether an IPv6 address is already in use on the local segment before it configures the address as its own using autoconfiguration. In essence, it provides a similar function to Gratuitous ARP used in IPv4. Solicited-Node Multicast addresses are defined by the IPv6 prefix FF02::1:FF00:0000/104. These addresses are comprised of the FF02::1:FF00:0000/104 prefix in conjunction with the low-order 24 bits of the Unicast or Anycast address. Figure 7.9 below illustrates the format of these IPv6 addresses:

Figure 7.9 – IPv6 Solicited-Node Multicast Addresses

 

In a manner similar to IPv4 Multicast mapping for Ethernet, IPv6 also uses a unique means to map Layer 3 IPv6 Multicast addresses to Layer 2 Multicast addresses. Multicast mapping in IPv6 is enabled by appending the low-order 32 bits of a Multicast address to the 16-bit prefix 33:33, which is the defined Multicast Ethernet prefix for IPv6 networks. This is illustrated in Figure 7.10 below for all the routers on the Interface-Local scope prefix FF02::2:

 

Figure 7.10 – IPv6 Multicast Addresses

 

Anycast Addresses

Anycast, which was introduced earlier in this section, can be described simply as one-to-nearest communication, because the nearest common address, based on routing protocol metrics, will always be preferred by the local device. In IPv6 there is no specially allocated range for Anycast, as Anycast addresses use Global Unicast, Site-Local, or even Link-Local addresses. However, there is an Anycast address reserved for special use. This special address is referred to as the Subnet-Router Anycast address and is formed with the subnet’s 64-bit Unicast prefix, with the remaining 64 bits set to zero (e.g., 2001:1a2b:1111:d7e5:0000:0000:000:0000). Anycast addresses must not be used as the source address of an IPv6 packet. These addresses are typically used by protocols such as Mobile IPv6, which is outside the scope of the CCNA.

 

Loopback Addresses

Loopback addresses in IPv6 are used in the same manner as in IPv4. Each device has one IPv6 Loopback address, which is comparable to the 127.0.0.1 Loopback address used in IPv4, and this address is used by the device itself. IPv6 Loopback addresses use the prefix ::1, which can be represented as 0000:0000:0000:0000:0000:0000:0000:0001 in the preferred address format. This means that in Loopback addresses, all bits are set to 0, except for the last bit, which is always set to 1. These addresses are always assigned automatically when IPv6 is enabled on a device and they can never be changed.

 

Unspecified Addresses

In IPv6 addressing, unspecified addresses are simply Unicast addresses that are not assigned to any interface. These addresses indicate the absence of an IPv6 address and are used for special purposes that include IPv6 DHCP and DAD. Unspecified addresses are represented by all 0 values in the IPv6 address and can be written using the :: prefix. In the preferred format, these addresses are represented as 0000:0000:0000:0000:0000:0000:0000:0000.