A. Understand the IPV6 multicast address
IPv6 multicast address is usually for IPv6 multicast service, while the core of a large number of IPv6 traffic using multicast, IPv6 is no longer broadcast, which communicate with a different IPv4, IPv6, multicast However, to understand,
We first need to understand three key points:
First, any node can be a multicast group members also called multicast group members;
Second, the source node may transmit the data packet to the multicast group,
Third, in a multi-node multicast group can receive the data sent to the multicast group.
For IPv6 multicast address is a fixed address prefix 8bit flag FF :: / 8,4bit of, 4bit and 112bit multicast scope multicast group identifier (group ID) composition.
| 8 | 4 | 4 | 112 |
+---------------+--------+---------+---------------------------------------------+
|11111111 | flgs | scop | group ID |
+---------------+--------+---------+----------------------------------------------+
Multicast Address High 8bit fixed value the FF , this high . 8 th bit in 4bit as flgs bit, 4bit to flooding scope multicast group.
FLGS bit: as 4bit : | 0 | R & lt | P | T |, FLGS high bit 1bit reserved, must be set to 0
T bit: If set to 0 indicates permanently assigned or well-known multicast address, if set to 1 indicates the address temporarily assigned dynamic and not fixed.
P bit: If set 1 , then this indicates a multicast address is based on a unicast prefix ipv6 multicast address. The default is 0 , if the P bit is set to 1 , the T bit must be 1 .
R bit: If set 1 , then this indicates a multicast address is embedded RP address ipv6 multicast address. The default is 0
II. Appreciated that the multicast address of the requesting node
Appreciated solicited-node multicast address of the IPv6 is important, because the solicited-node multicast address of the IPv6 increases the data link layer address resolution efficiency, the IPv4 environment, the communication need to obtain an IP address and MAC address of the target host, when uses the ARP protocol to know the IP address of the target to resolve the MAC address of the data link layer, the ARP address request message is transmitted using broadcast, but no longer in a broadcast application in IPv6 environments, it abandoned the use of the ARP protocol, Instead ICMPv6 request message to the node two issues now require in-depth understanding of:
n node requesting host is how efficient to replace IPv4, ARP protocol?
n node requesting host is how to implement the technology? IPv6 multicast address how this technology is generated?
1) In the ARP Address Resolution Protocol IPv4 environment using a broadcast destination address (255.255.255.255 or FFFF.FFFF.FFFF) transmits the MAC address request message to all hosts on the entire Ethernet link, even if the host only A D request the host MAC, B and C the host is the host receives a broadcast request from a performance and efficiency perspective this is obviously not scientific. Therefore, in an IPv6 environment abandon way broadcasting, instead of using the multicast MAC address resolution request in the form of point to point directly to the requesting node multicast address of a multicast host D FF02 :: 1: FFAA: 4C3E . And no longer request message to the host independent of host B and C, the node is indeed the requested IPv6 efficient ARP protocol to replace IPv4.
2)节点请求主机能够以一种“点对点”的形式将MAC地址请求消息发送到目标主机D，是因为D主机的请求节点组播地址FF02::1:FFAA:4C3E在整个链路上是唯一的，所以源主机可以直接将节点请求信息发到目标主机节点请求组播地址上，节点请求组播地址是提高MAC地址解析的核心技术，那么这个目标主机的节点请求组播是如何生成的？
节点请求组播的构成如下图12.60所示，该组播地址是将一个单播IPv6地址（可以是本地链路地址也可以是全球单播地址）的后24位填充到一个请求节点组播地址的124位的固定前缀FF02::1:FF中，生成一个唯一的节点请求组播地址。
三. 理解IPV6的组播地址与MAC地址的映射关系
IPv6的节点请求多播地址去替代了ARP地址解析协议的目标广播地址255.255.255.255，注意：这只能表示IPv6的节点请求从网络层上的优化与改进，在数据链路层会有什么样改进这是本小节讨论的重点。
在 IPv4的ARP地址解析协议中的请求消息报文的目标链路层地址为FFFF.FFFF.FFFF哪么在IPv6中节点请求消息报文的目标链路层地址是什 么？在IPv4中组播的IP地址是可以映射成为数据链路层的MAC地址，在IPv6中这个概念仍然生效，IPv6的节点请求组播地址也可以映射成数据链路 层地址，具体方法如下图12.62所示组播的MAC地址是通地IPv6的组播IP地址映射而来，映射的原则是：将IPv6组播地址的后32位取出，填充到固定前缀是3333的MAC地址中来生成数据链路层地址。
比如一个众所周知的IPv6组播地址：FF02::1它所对应的MAC地址是什么？首先将该“零压缩”的IPv6地址还原为完整格式的IPv6地址为：FFF02:0000:0000:0000:0000:0000: 0000: 0001，然后将该地址的最后32个二进制位取出，一个十六进制位就为4个二进制位，所以最后32个二进制位就是0000:0001,将这部分填充到IPv6组播MAC的固定前缀3333,就得到FF02::1所对应的MAC地址是3333:0000:0001。具体过程如下图12.63所示。
