Information Technology Reference
In-Depth Information
Yo u c a n u s e t h e a n y c a s t a d d r e s s t o i d e n t i f y a s e t o f r o u t e r s o r s e r v e r s w i t h i n a n a r e a .
When a packet is sent to the anycast address, it is delivered to the nearest device as deter-
mined by the routing protocol. An example of the use of anycast addresses is to assign an
anycast address to a set of servers—one in North America, and the other in Europe. Users
in North America would be routed to the North American server, and those in Europe to
the European server.
Yo u c a n n o t u s e a n a n y c a s t a d d r e s s a s a s o u r c e a d d r e s s . A l s o , y o u m u s t e x p l i c i t l y c o n f i g -
ure nodes to which the anycast address is assign to recognize the anycast address.
IPv6 Multicast Addresses
The IPv6
multicast
(one-to-many) address identifies a set of hosts. The packet is delivered
to all the hosts identified by that address. This type is similar to IPv4 multicast (Class D)
addresses. IPv6 multicast addresses also supersede the broadcast function of IPv4 broad-
casts. You use an “all-nodes” multicast address instead. One additional function of IPv6
multicast is to provide the IPv4 broadcast equivalent with the all-nodes multicast group.
Key
To p i c
Some IPv6 multicast addresses are
FF01:0:0:0:0:0:0:1—Indicates all-nodes address for interface-local scope.
FF02:0:0:0:0:0:0:2—All-routers address for link-local.
RFC 4291 specifies the format of IPv6 multicast addresses. As shown in Figure 9-6, the
fields of the IPv6 multicast address are the FP, a value of 0xFF, followed by a 4-bit flags
field, a 4-bit scope field, and 112 bits for the group identifier (ID). Again, a quick way to
recognize an IPv6 multicast address is that it begins with FF::/8.
8 Bits
4 Bits
4 Bits
112 Bits
11111111
FLGS
SCOP
Group ID
Figure 9-6
Multicast Address Format
The FLGS (flags) field consists of three leading 0s followed by a T bit: 000T. If T = 0, the
address is a well-known multicast address assigned by the global IANA. If T = 1, the ad-
dress is not a permanently assigned address.
The SCOP (scope) field limits the scope of the multicast group. Table 9-4 shows the as-
signed scope values.
Ta b l e 9 - 4
Multicast Scope Assignments
SCOP (Binary)
SCOP (Hexadecimal)
Assignment
0000
0
Reserved
0001
1
Node-local scope
0010
2
Link-local scope
