Multicast Addresses
Figure 4-11 shows the types of multicast addresses. Multicast is a technique in which a device sends a single packet to multiple destinations simultaneously (one-to-many). (Remember that a unicast address sends a single packet to a single destination [one-to-one].) Multiple destinations can actually be multiple interfaces on the same device, but they are typically different devices.
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Figure 4-11 Multicast Addresses
An IPv6 multicast address defines a group of devices known as a multicast group. IPv6 multicast addresses use the prefix ff00::/8, shown in Table 4-10, which is equivalent to the IPv4 multicast address 224.0.0.0/4. A packet sent to a multicast group always has a unicast source address. A multicast address can never be the source address. Unlike IPv4, there is no broadcast address in IPv6. Instead, IPv6 uses multicast, including an all-IPv6 devices well-known multicast address and a solicited-node multicast address.
Table 4-10 IPv6 Multicast Address Representations
| Representation | IPv6 Multicast Address |
| Preferred | ff00:0000:0000:0000:0000:0000:0000:0000/8 |
| Leading 0s omitted | ff00:0:0:0:0:0:0:0/8 |
| Compressed | ff00::/8 |
Figure 4-12 shows the structure of an IPv6 multicast address. The first 8 bits are 1-bits (ff), followed by 4 bits allocated for flags and a 4-bit Scope field. The Scope field defines the range to which routers can forward the multicast packet. The next 112 bits represent the Group ID.
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Figure 4-12 IPv6 Multicast Address
The 4 bits following 1111 1111 represent four different flags. The first three flags, 0 (reserved), R (rendezvous point), and P (network prefix), are beyond the scope of this book. The fourth flag, the least significant bit (LSB), or rightmost bit, is the transient flag (T flag). The T flag denotes the two types of multicast addresses:
Permanent (0): These addresses, known as predefined multicast addresses, are assigned by IANA and include both well-known and solicited multicast.
Nonpermanent (1): These are “transient” or “dynamically” assigned multicast addresses. They are assigned by multicast applications.
As shown in Figure 4-11, there are two types of predefined multicast addresses, both of which use the Flag field with a 0x0 value:
Well-known multicast addresses
Solicited-node multicast addresses
Well-Known Multicast Addresses
Well-known multicast addresses have the prefix ff00::/12. As shown in Figure 4-12, this means that the third hexadecimal digit, the Flag field, is always set to 0. Well-known multicast addresses are predefined or reserved multicast addresses for assigned groups of devices. These addresses are equivalent to IPv4 well-known multicast addresses in the range 224.0.0.0 to 239.255.255.255. Some examples of IPv6 well-known multicast addresses include the following:
ff02::1: All IPv6 devices
ff02::2: All IPv6 routers
ff02::5: All OSPFv3 routers
ff02::a: All EIGRP (IPv6) routers
Solicited-Node Multicast Addresses
Solicited-node multicast addresses are used as a more efficient approach to IPv4’s broadcast address. As discussed in Chapter 2, the solicited-node multicast is used in Layer 3-to-Layer 2 address resolution, similar to how Address Resolution Protocol (ARP) is used in IPv4. Solicited-node multicast addresses are automatically created using a special mapping of the device’s unicast address with the solicited-node multicast prefix ff02:0:0:0:0:1:ff00::/104. Solicited-node multicast addresses are automatically created for every unicast address on a device.