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Chapter 3 IPv6 Addressing 55 routing policy for the organization network.Once assigned,routers on the IPv6 Internet forward IPv6 traffic matching the 48-bit prefix to the routers of the organization's site. Subnet ID The Subnet ID is used within an organization's site to identify subnets within its site.The size of this field is 16 bits.The organization's site can use these 16 bits within its site to create 65,536 subnets or multiple levels of addressing hierarchy and an efficient routing infrastructure.With 16 bits of subnetting flexibility,a global unicast pre fix assigned to an organization site is equivalent to a public IPv4 Class A address prefix (assuming that the last octet is used for identifying nodes on subnets).The routing struc- ture of the organization's network is not visible to the ISP. Interface ID Indicates the interface on a specific subnet within the site.The size of this field is 64 bits.The interface ID in IPv6 is equivalent to the node ID or host ID in IPv4. Trillions of Sites Another way to gauge the practical size of the IPv6 address space is to examine the num- ber of sites that can connect to the IPv6 Internet.With the current allocation practice defined in RFC 3587 of 48-bit global address prefixes,it is possible to define 245 or 35,184,372,088,832 possible 48-bit prefixes to assign to sites connected to the IPv6 Internet.There are more IPv6 sites than possible IPv4 addresses.This large number of sites is possible even when we are using only one-eighth of the entire IPv6 address space. By comparison,using the Internet address classes originally defined for IPv4,it was pos- sible to assign 2,113,389 address prefixes to organizations connected to the Internet. The number 2,113,389 is derived from adding up all the possible Class A,Class B,and Class C address prefixes and then subtracting the prefixes used for the private address space.Even with the adoption of CIDR to make more efficient use of unassigned Class A and Class B address prefixes,the number of possible sites connected to the Internet is not substantially increased,nor does it approach the number of possible sites that can be connected to the IPv6 Internet. Topologies Within Global Addresses The fields within the global address create a three-level topological structure,as shown in Figure 3-2. -001 Global Routing Prefix Subnet ID Interface ID 48 bits 16 bits 64 bits Public Topology Site Interface ldentifier Topology Figure 3-2 The topological structure of the global address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`a_^:_ <!" " *)%!!" 4# #**")) "#"#!""3 "." # ) "+#))!% V"A3X' b9 cd efghijikilmnokphqrnhrqgishfglkitokouuqgpp vwxyz{ |}x~y~
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