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IPv6 Address Representation and Address Types

Chapter Description

In this chapter from IPv6 Fundamentals: A Straightforward Approach to Understanding IPv6, 2nd Edition, author Rick Graziani examines all the different types of IPv6 addresses in the unicast, multicast, and anycast categories.

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.

Figure 4-11

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.

Figure 4-12

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.

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