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IPv6 Global Aggregatable Unicast Addresses

As has been repeated several times in this book, scalability is an emergent property of summarization. This is just as true for IPv6 as for IPv4. As in IPv4, the far left bits of IPv6 addresses indicate the routing prefix and may be summarized. Theoretically, there are 264 IPv6 prefixes. If each prefix were stored in router memory using 256 bits (32 bytes), then the routing table would consume 5.9 * 1020 bytes! Therefore, addresses must be deployed hierarchically and summarized, or the number of networks could grow to be too large for routers to track. 
Figure 20-3 shows the IPv6 global-aggregatable unicast address structure as described by RFC 3587, IPv6 Global Unicast Address Format. The IPv6 global unicast address is similar to the IPv4 global unicast address. 

Figure 20-3. IPv6 Global Aggregatable Unicast Address Structure 

The first 48 bits of the IPv6 global unicast address are used for global routing at the Internet service provider (ISP) level. The next 16 bits are the subnet ID, allowing an enterprise to subdivide their network. The final 64 bits are the interface ID, typically in EUI-64 format. The Internet Assigned Numbers Authority (IANA) is currently assigning addresses that start with the binary value 001, which is 2000::/3, for IPv6 global unicast addresses. This is one-eighth of the total IPv6 address space. The IANA is currently allocating address space in the 2001::/16 ranges to the registries. Registries typically have a /23 range, and allocate /32 ranges to ISPs. 
For example, an ISP might assign 2001:0:1AB::/48 to an organization. In a network assigned subnet 5, the prefix would be 2001:0:1AB:5::/64. On a device with a MAC address 00-0F-66-81-19-A3, the EUI-64 format interface ID would be 020F:66FF:FE81:19A3. The complete IPv6 global unicast address of the device would therefore be 2001:0:1AB:5:20F: 66FF:FE81:19A3. 
Note 
In the now obsolete RFC 2374, An IPv6 Aggregatable Global Unicast Address Format, two other fields were included in the global prefix: the Top-Level 
Aggregator and the Next-Level Aggregator. Some early IPv6 networks may still use these fields, but they are no longer included in the latest RFC 3587, IPv6 Aggregatable Global Unicast Address Format.  

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