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IPv6 is an extension of IP with several advanced features:
Larger address space
Simpler header
Autoconfiguration
Extension headers
Flow labels
Mobility
"Baked in" security
Of these, many capabilities have been backported to IPv4. The primary adoption of IPv6 will be driven by the need for more addresses. Given the growth in Internet use and the emergence of large groups of Internet users in developing countries, this is a significant requirement.
IPv6路由前缀
IPv4地址长32位，而IPv6地址是128位。IPv6地址由以下元素组成（参见图8-1.):
单播的前三位（/ 3）始终001。
The next 13 bits (/16) are Top-Level Aggregator (TLA) the upstream ISP.
The next 24 bits (/40) are the next-level aggregator or regional ISP.
Enterprises are assigned /48 and have 16 bits of subnetting.
RFC 2374 IPv6地址结构
IPv6接口ID
The host portion of the address is last 64 bits. It can be assigned manually, using DHCP v6, or using stateless autoconfiguration.
An end-system uses stateless autoconfiguration by waiting for a router to advertise the local prefix. If the end system has a 64-bit MAC, it concatenates the prefix and its MAC to form an IPv6 address. If the end system has a 48-bit MAC, it flips the global/local bit and inserts 0xFFEE into the middle of the MAC. The resulting 64-bit number is called the EUI64. The prefix and EUI64 are concatenated to form the address.图8-2.shows how a host uses its MAC address to create its IPv6 address.
EUI64.
Simplified Presentation of IPv6 Address
有两种方法可以缩短IPv6地址的表示。采取示例地址4001：0000：0001：0002：0000：0000：0000：ABCD。
Leading zeros may be omitted. This makes the example 4001:0:1:2:0:0:0:ABCD.
序列零可以显示为双冒号一次。这使例4001：0：1：2 :: ABCD。
IPv6标题
The IPv6 header is similar to the IPv4 header. The largest changes have to do with the larger addresses, aligning fields to 64-bit boundaries and moving fragmentation to an extension header.
IPv6标题
The fields are:
Version—6.
优先级类似于版本4中的DSCP，该八位字段用于描述相对优先级。
Flow—20-bit flow label allows tagging in a manner similar to MPLS.
长度 - 数据包中数据的长度。
下一个标题 - 表示应该解释IP标头后的位。可以表示TCP或UDP，或者它可以显示一个扩展标题。
Hop Limit—Similar to TTL.
Source and Destination—IPv6 addresses.
可以遵循零或更多的延伸标题，包括：
逐跳选项 - 中间设备的选项。
Destination options—Options for the end node.
Source routing—Specifies "way stations" that the route must include.
碎片化 - 用于划分包。
Authentication—Used to attest to source. Replaces the AH header from IPSec.
Encryption—Replaces the IPSec ESP header.
Advanced Features
“高级”功能是IPv4中不可用的元素，或者已显着改变。例如，重要的是要知道广播的想法已被抛弃，并且设备通常会响应一组IPv6地址。
Specifying Destinations
IPv6不支持广播，但替换具有多播的广播。IPv6还使用涉及在两个设备上使用相同的地址。yourcast可用于实现冗余，并已被推到IPv4。
指定来源
Each IPv6 system must recognize the following addresses:
Unicast address
Link local address (FE80/10 | EUI64)
环回（:: 1）
全节点组播（FF00 :: 1）
Site-local multicast (FF02::2)
Solicited-nodes multicast (FF02::1:FF00/104)
此外，某些系统还将使用以下地址：
IPv4映射地址（0 :: ffff | 32位，IPv4地址）。
与另一个系统共享的第二个单播地址（yourcast）。
Additional multicast groups.
路由器必须支持子网路由器yourcast（所有零eui64）。
Routers must support local all-routers multicast (FF01::2), link-local (FF02::2), and site-local (FF05:2).
路由器必须支持路由协议组播组。
重新编号
IPv6 supports easy network renumbering. A router sends out a "router advertisement" with a new prefix and a token that instructs end systems to perform stateless autoconfiguration. Hosts then recognize the command and update their addresses.
Anyone who has had to renumber a large range of IPv4 addresses can testify to what a boon this feature will be!
Mobility
IPv6还包括更好地支持漫游系统。使用IPv6移动性，漫游者与“家庭代理”保持联系，即他们的家庭路由器。将代理转发到“家庭地址”的流量将代理转发到当前地址。然后漫游者将绑定更新发送到其相应的代理，以便将来的流量直接发送到漫游地址。
IPv6 Routing
IPv6 is not enabled by default on Cisco routers. To enable IPv6 routing, the command isRouter(config)#IPv6单播路由.
启用IPv6后，将地址分配给接口，就像版本4一样：
路由器（Config-If）＃IPv6地址前缀/前缀长度
To make this less abstract, a more complete example that shows an IPv6 implementation is shown in Example 8-1.
示例8-1启用IPv6路由和分配地址
RouterA#configure terminalRouterA(config)#IPv6单播路由RouterA(config)#interface fastethernet0/0RouterA(config-if)#description Local LANRouterA(config-if)#ipv6 address 4001:0:1:1::2/64RouterA(config-if)#interface serial 1/0RouterA(config-if)#描述点对点线到互联网RouterA(config-if)#ipv6 address 4001:0:1:5::1/64
静止路由
Static routing with IPv6 works exactly like it does with version 4. Aside from understanding the address format, there are no differences. Static routes are not currently on the BSCI test. The syntax for the IPv6 static route command is shown below, and Example 8-2 is supplied so that the command may be viewed in context as it might be applied.
Router(config)# ipv6 route ipv6-prefix/prefix-length {ipv6-address | interface-type interface-number [ipv6-address]} [administrative-distance] [administrative-multicast-distance | unicast | multicast] [tag tag]
Example 8-2 Configuring Static IPv6 Routes
RouterA(config)#IPv6路线4001：0：1：2 ::/ 64 4001：0：1：1 :: 1RouterA(config)#IPv6路由::/ 0 Serial1 / 0
为IPv6 ripng
RIPng是RIP的IPv6，并在RFC 2080中定义.IPv4的RIPv2，RIPng是一个距离矢量路由协议，它使用跳数为其度量，并且具有15的最大跳数。RIPng也使用周期性多播更新 - 每个跳数也使用周期性组播更新30秒 - 宣传路线。多播地址是FF02 :: 9。
RIPng is not on the BSCI exam at present, but it is presented here for completeness and to round out your appreciation for IPv6 routing and to prepare the reader for trial implementations of IPv6.
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