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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 3 of 5)

Original Division of the Global Routing Prefix: Aggregators

The global routing prefix is similarly divided into a hierarchy, but one that has been designed for the use of the entire Internet, a la CIDR. There are 45 bits available here (48 bits less the first three that are fixed at “001”), which is a lot. When the unicast address structure was first detailed in RFC 2374, that document described a specific division of the 45 bits based on a two-level hierarchical topology of Internet registries and providers. These organizations were described as:

  1. Top-Level Aggregators (TLAs): The largest Internet organizations, which were to be assigned large blocks of IPv6 addresses from registration authorities.

  2. Next-Level Aggregators (NLAs): These organizations would get blocks of addresses from TLAs and divide them for end-user organizations (sites).

The 45 bits were split between these two uses, with a few bits reserved in the middle to allow expansion of either field if needed. Thus, the RFC 2374 structure for the 45 bits was as shown in Table 63:

Table 63: Historical IPv6 Unicast Routing Prefix Structure

Field Name

Size (bits)




Top-Level Aggregation Identifier: A globally-unique identifier for the Top-Level Aggregator. There are 13 bits so there were a maximum of 8,192 TLAs allowed.



Reserved: These 8 bits were reserved for future use and set to zero. By leaving these 8 bits between the TLA ID and NLA ID unused, they could be later used to expand either the TLA ID or NLA ID fields as needed.



Next-Level Aggregation Identifier: Each TLA was given this 24-bit field to generate blocks of addresses for allocation to its customers. The NLA ID is unique for each TLA ID. The use of the 24 bits was left up to the TLA organization.

You'll notice my use of the past tense in the description of the TLA/NLA structure, and that table heading is a pretty big giveaway too. In August 2003, RFC 3587 was published, which in a nutshell says “uh, never mind about all that TLA/NLA stuff”. J The decision was made that having this structure “hard-coded” into an Internet standard was inflexible, and it made more sense to allow the regional Internet registries (APNIC, ARIN, LACNIC and RIPE) decide for themselves how to use the 45 bits.

Note: The obsoleting of the “TLA/NLA” structure occurred after many years of people getting used to it, so for some time to come you will still routinely see mention of those terms in IPv6 descriptions. (This is why I included discussion of them here.)

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IPv6 Address Space Allocation
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IPv6 Interface Identifiers and Physical Address Mapping
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Version 3.0 - Version Date: September 20, 2005

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