186 Part Two IPv6 Protocols
The most local scope, interface-local, specifies a single network interface
to be used to transmit multicast packets over the loopback address. The
scope can expand to the local link, site, organization, or global IPv6 Inter-
net. As shown in Table 10–1, the all-zeros (0) and all-ones (0xF) values
are reserved, with half of the rest of the values unassigned (but available
for administrators who want to assign their own regions). Site-local and
link-local scopes correspond to the unicast definitions for those scopes (see
preceding section).
Beyond the link- and site-local scopes that can be automatically defined
by physical connectivity lie the admin-local and organization-local scopes
that are administratively configured and can span separate networks. The
organization-local scope is meant to be able to cross site boundaries within
a single organization.
Most important is the group ID field, which identifies the multicast group
within the group’s defined scope. As with IPv4 multicast groups, there
are permanently assigned IPv6 multicast groups whose IDs have specific
meaning. For instance, permanently assigned group IDs of the form ::2
specify “all routers” within the defined scope. So the following addresses
specify three different multicast groups that consist of all the IPv6 routers
in different scopes.
The first group includes IPv6 routers within scope 1 (the local interface),
the second includes all routers within scope 2 (the local link), and the
third includes all routers within scope 5 (site-local). If the high-order octets
were FF0E, the group (if it was used) would consist of all IPv6 routers in
the global IPv6 Internet. Those packets, however, would not likely be
forwarded throughout the IPv6 Internet.
10.3 Reserved and Permanent Multicast Addresses
The group ID has meaning for permanently assigned groups, but tran-
sient group IDs have meaning only within their scope. The same transient,
Chapter 10 IPv6 Multicast 187
site-local, multicast address may be used at any number of separate sites.
And FF15::101, a transient site-local address, does not have any relation
to the similar permanent site-local scope address FF05::101.
There are other reserved and permanent multicast address allocations. For
example, under no circumstances is the “all-zeroes” address a valid one
for multicast, no matter what the scope. RFC 3513 explicitly reserves these
addresses and states that can never be used.
This list of reserved addresses should be filtered on all IPv6 routers, just
to ensure that packets sent to or from those addresses are not forwarded.
Another set of multicast addresses that have been permanently assigned
are the All Nodes Addresses for interface-local and link-local scopes.
FF01:0:0:0:0:0:0:1 all IPv6 nodes, within scope 1
188 Part Two IPv6 Protocols
FF02:0:0:0:0:0:0:1 all IPv6 nodes, within scope 2
These groups are defined as consisting of all IPv6 nodes, within the scope
The All Routers Addresses are defined as follows.
FF01:0:0:0:0:0:0:2 all IPv6 routers,
within scope 1 (interface-local)
FF02:0:0:0:0:0:0:2 all IPv6 routers,
within scope 2 (link-local)
FF05:0:0:0:0:0:0:2 all IPv6 routers,
within scope 5 (site-local)
The Solicited-Node Address is used with ICMPv6 to determine whether an
IPv6 node is configured with a particular IPv6 address. All nodes are
required to subscribe to the solicited-node address for every unicast and
anycast address that node responds to. This is the format of this address.
Solicited-node multicast addresses are computed as a function of the node’s
unicast and anycast addresses; the address is formed by taking the low-
order 24 bits of an address (unicast or anycast) and appending those bits
to the prefix FF02:0:0:0:0:1:FF00::/104, resulting in a multicast address in
the following range.
Solicited-node addresses and their use are discussed in the next section.
Fixed-scope multicast addresses are permanently assigned over specific
scope values. The most current assignments are available through the
IANA Web site; those available as of mid-2003 are listed in Table 10–2.

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