The TCP/IP Guide - IPv6 Global Unicast Address Format

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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)	Description
*(Unicast Indicator)*	3	Each unicast address starts with “001”; there is no official name for this (it used to be called the Format Prefix.
*Level1 ID*	10	*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*	12	*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*	23	*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)!