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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)
                     9  IPv6 Addressing

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IPv6 Address Space Allocation
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IPv6 Interface Identifiers and Physical Address Mapping
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IPv6 Global Unicast Address Format
(Page 4 of 5)

An Example Division of the Global Routing Prefix Into Levels

So, there is no longer any single structure for determining how the 48-bit routing prefix is divided in the global unicast hierarchy. As one example, it might be possible to divide it into three levels, as shown in Table 64, and diagrammed in Figure 97.

Table 64: Example IPv6 Unicast Routing Prefix Structure

Field Name

Size (bits)


(Unicast Indicator)


Each unicast address starts with “001”; there is no official name for this (it used to be called the Format Prefix.

Level1 ID


Level 1 Identifier: The identifier of the highest level in the hierarchy. This would be used for assigning to the biggest Internet organizations the largest blocks of addresses in the global hierarchy. The number of Level 1 organizations would be 210 or 1,024.

Level2 ID


Level 2 Identifier: Each block assigned to a Level 1 organization would use 12 bits to create 4,096 address blocks to divide amongst the lower-level organizations it serves.

Level3 ID


Level 3 Identifier: Each Level 2 organization has 23 bits to use to divide its Level 2 address block. Thus, it could create over 8 million individual “/48” address blocks to assign to end user sites. Alternately, the 23 bits could be divided further into still lower levels to reflect the structure of the Level 2 organization's customers.

Figure 97: Example IPv6 Unicast Routing Prefix Structure

The top row shows the global IPv6 unicast address format. The second shows one example way to divide the Global Routing Prefix, into three levels using 10, 12 and 23 bits respectively. The third row shows how the first 10 bits are used to create 210 or 1,024 different Level1 blocks. The next row illustrates that for each of these 13-bit prefixes, we could have 212 or 4,096 Level2 blocks. Then, within each 25-bit Level2 ID, we have 23 bits or 8,388,608 Level3 blocks. At the bottom, a Level3 or “/48” would be assigned to an individual organization.


This is just one possible, theoretical way that the bits in a “/48” network address could be assigned. As you can see, with so many bits there is a lot of flexibility. In the scheme above we can have over 4 million level 2 organizations, each of which can assign 8 million /48 addresses. And each of those is equivalent in size to an IPv4 Class B address (over 65,000 hosts)!

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Version 3.0 - Version Date: September 20, 2005

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