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IPv6 Multicast and Anycast Addressing
(Page 1 of 5)
One of the most significant modifications
in the general addressing model in IPv6 was a change to the basic types
of addresses and how they were used. Unicast addresses are still the
choice for the vast majority of communications as in IPv4, but the bulk
addressing methods are different in IPv6. Broadcast as a specific addressing
type has been eliminated. Instead, support for multicast addressing
has been expanded and made a required part of the protocol, and a new
type of addressing called anycast has been implemented.
IPv6 Multicast Addresses
Let's start by looking at multicast
under IPv6. Multicasting is used to allow a single device to send a
datagram to a group of recipients. IPv4 supported multicast
addressing using the Class D address block
in the classful addressing scheme. Under IPv6, multicast
addresses are allocated from the multicast block. This is 1/256th of
the address space, consisting of all addresses that begin with 1111
1111. Thus, any address starting with FF in colon
hexadecimal notation is an IPv6 multicast address.
The remaining 120 bits of address
space are enough to allow the definition of, well, a gazillion or three
multicast addresses. (Okay, it's officially about 1.3 trillion trillion
trillion addresses.) Much the way the allocation of unicast addresses
was organized by using a special
format to divide up these many bits, the
same thing was done for multicast addresses.
IPv6 Multicast Address Format
The format for multicast addresses
is explained in Table 65
and illustrated in Figure 101.
Table 65: IPv6 Multicast Address Format
The first eight bits are always
1111 1111 to indicate a multicast address. This used to
be called the Format Prefix before the term was dropped
as explained in the
topic on IPv6 address space allocation;
the field now has no name.
Four bits are reserved for flags that can be used to indicate the nature
of certain multicast addresses. At the present time the first three
of these are unused and set to zero. The fourth is the T
(Transient) flag. If left as zero, this marks the multicast address
as a permanently-assigned, well-known multicast address,
as we will see below. If set to one, this means this is a transient
multicast address, meaning that it is not permanently assigned.
ID: Defines a particular group within each scope level.