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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
- Top-Level Aggregators (TLAs):
The largest Internet organizations, which were to be assigned large
blocks of IPv6 addresses from registration authorities.
- 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
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.
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.
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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Version 3.0 - Version Date: September 20, 2005
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