Multicast Addresses

Multicast addresses identify groups of interfaces, each of which can contain multiple multicast addresses. Multicast addresses are distinguishable from unicast addresses because they always begin with OxFF. There is no such thing as broadcasting at the network layer in IPv6. Broadcasting induced a lot of extra overhead on nodes that were not necessarily interested in the broadcast packet. All IP interfaces that received a broadcast packet had to process the packet to see whether it might be the intended recipient. Very often, the node was not the intended recipient. Every IPv6 interface knows the multicast groups to which it belongs. A multicast packet is processed only by those interfaces that belong to the multicast group. IPv6 uses multicasting rather than broadcasting.

IPv6 multicast addresses may be either assigned by an official addressing authority (well-known addresses) or transient—that is, locally assigned for nonglobal use. The initial assignment of IPv6 multicast addresses was based on assigned IPv4 multicast addresses. All relevant IPv4 multicast addresses are converted to IPv6 multicast addresses. You can find a complete list of the currently assigned IPv6 multicast addresses in RFC 2375. Tabic 8-6, shown in a moment, lists some examples.

IPv6 multicast addresses are also scoped. The addresses have a field that identifies the scope as either local to the node, local to the link, local to the site, local to the organization, or global. Transient addresses defined within a particular scope are meaningful only to nodes within that scope. The same address may be defined in a different scope, or a different network, and have a completely different meaning.

Figure 8-9 illustrates the format of a multicast address.

Figure 8-9 Multicast Address Format




112 bits




Group ID

The leading octet, 11111111, identifies this address as multicast.

figs is a set of 4 bits. The leading 3 bits are reserved and must be set to 0. The last bit indicates whether the multicast address is an address permanently assigned by the global Internet numbering authority or whether it is not permanently assigned, known as "transient." A value of 0 in the fourth bit indicates that the multicast address is "well-known." The global Internet numbering authority assigned it.

scop is a 4-bit value used to limit the scope of the multicast address. Table 8-5 lists the values.

Table 8-5 Multicast Address Scope Values






Node local scope


Link local scope


Site local scope

8 Organization local scope


Global scope

f Reserved f Reserved

Any of the scope values may exist with either well-known or transient addresses. Transient addresses within a given scope are valid only for that scope. They are meaningless under any other scope.

Group ID identifies the multicast group, either well-known or transient, within the given scope.

Table 8-6 lists some common multicast groups, along with their assigned addresses and scopes.

Table 8-6 Some IPv6 Weil-Known Multicast Addresses

IPv6 Well-Known Multicast Address

IPv4 Well-Known Multicast Address

Multicast Group

Node-Local Scope


All-nodes address


All-routers address

Link-Local Scope


All-nodes address

Table 8-6 Some IPv6 Well-Known Multicast Addresses (Continued)


All-routers address




OSPFIGP-designated routers


RIP routers



All PIM routers

Site-Local Scope


All-routers address

Any Valid Scope


Network Time Protocol (NTP)





Upon interface initialization and when multicast protocols and applications are initialized, nodes join the required multicast groups. Nodes join the all-nodes multicast addresses of FF01::1 and FF02::1. The address' formats indicate that the multicast addresses are well-known and have node-local and link-local scope, respectively. Routers join the all-routers multicast address of FF01::2, FF02::2, and FF05::2. These are well-known addresses with node-local, link-local, and site-local scope, respectively.

You can see from Table 8-6 that a multicast group capable of operating within multiple scopes has multiple IPv6 addresses. This is not the case with the well-known IPv4 addresses. These are not scoped. Two methods of multicast scoping with IPv4 were discussed in Chapter 5, "Introduction to IP Multicast Routing." TTL scoping requires the network administrator to set TTL thresholds on multicast boundaries. If the TTL value in a multicast packet is lower than the defined threshold when the packet reaches the boundary, the packet is discarded. One drawback to this approach is its inflexibility—an interface's TTL threshold applies to all multicast packets exiting the interface. Another drawback is that in a large network, it is difficult to predict what the correct TTL threshold value should be. The other type of scoping for IPv4, administrative scoping, defines a range of private use multicast addresses that can be used to define scope within an enterprise. The reserved range is Suggested scoping ranges are for local or site scope and for organizationwide scope. These are just suggested ranges. Enterprises are free to use the addresses as they see fit. Administrative scoping might work fine within an organization, but it cannot work globally. The addresses are lor private use only.

Multicast scoping is built into all IPv6 multicast addresses. Currently, five levels of scopr are defined. Link-local scope is achieved in IPv4 with TTL scoping by setting the TTL

value in all link-local multicast packets to 1. The rest of the IPv6 scopes create the potential for various levels of multicast containment. Multicast applications using IPv6 can be contained within a link, site, or organization. There are reserved addresses for future scopes. Scoped, well-known multicast addresses enable multicast containment while at the same time ensuring that the same address is not used for two different multicast groups. There is no danger of two companies using the same address for two different applications and then conflicting when the two companies later decide to merge.

A particular type of multicast address is the "solicited-node" address. Solicited-node multicast addresses are used by various IPv6 functions to communicate with IPv6 nodes. The functions' use of the address is discussed in the section "IPv6 Functionality." Solicited-node multicast addresses are created and assigned for every unicast and anycast address assigned to an interface, other than the link-local address. The solicited-node multicast address is created using the last 24 bits of the interface ID and appending it to the prefix FF02:0:0:0:0:1:FF00::/104. Figure 8-10 illustrates how a solicited-node multicast address is formed.

igure 8-10 Solicited-Node Multicast Address Formation

Prefix: FEC0::/64

Ethernet MAC: 0000.0C0A.2C51 EUI-64: ::200:CFF:FE0A:2C51 Site-local: FEC0::200:CFF:FE0A:2C51

Solicited-node multicast:FF02::1 :FF0A:2C51

An interface with the MAC address 0000.0C0A.2C51 forms EUI-64 interface ID ::200:CFF:FE0A:2C51, which then creates link-local address FE80::200:CFF:FE0A:2C51. The site-local prefix FEC0::/64 on subnet 0 creates site-local address FEC0::200:CFF: FE0A:2C51. A solicited-node multicast address is formed because the interface now has a site-local address. The solicited-node address takes the last 24 bits of the interface ID, 0A:2C51, and appends it to the solicited-node prefix, forming FF02::1:FF0A:2C51.

Each interface may have multiple prefixes and multiple IPv6 addresses associated with it. The interface ID is likely the same for all addresses. Creating the solicited-node multicast address out of the final 24 bits of the interface ID minimizes the number of multicast addresses that the node must join.

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