SLA ID   Indicates the Site Level Aggregator (SLA) for the address.

The SLA ID is used by an individual organization to identify subnets

within its site. The size of this field is 16 bits. The organization can

use these 16 bits to create 65,536 subnets or multiple levels of

addressing hierarchy and an efficient routing infrastructure. With 16

bits of subnetting flexibility, an aggregatable global unicast prefix

assigned to an organization is equivalent to that organization being

allocated an IPv4 Class A network ID (assuming that the last octet is

used for identifying nodes on subnets). The structure of the cus 

tomer's network is transparent to the ISP.

  Interface ID   Indicates the interface on a specific subnet. The size of

this field is 64 bits.

With IPv6, small to regional network service providers and end users

will no longer have the ability to obtain address space directly from reg 

istries. Instead, TLAs will be assigned address blocks, which they will

in turn be in charge of managing and delegating to their downstream

connections (NLAs and SLAs). This shift in address management is

thought to be much more efficient than the current address management

policies. This policy places the burden on backbone TLA core providers

to carry the routes as transit. With this new hierarchical architecture,

the number of Internet Core routing entries that need to be carried is

decreased, thus limiting the scope of future Internet routing complex 

ity. Figure 38 depicts a generic addressed IPv6 Internet.

F I G U R E   3 8 .   R o u t i n g   A d v e r t i s e m e n t s   w i t h   I P v 6

Figure 38 shows two TLAs, and a variety of NLAs, and SLAs in various

configurations. TLA I owns a Top Level Aggregator block. TLA I is

assigned 3FFE:2900::/24 as its TLA delegation, and TLA II is assigned

3FFE:4200::/24 as its TLA delegation. TLA I and TLA II must supply

each other with these routes for routing to operate properly between

TLA I and TLA II backbones. 

TLA I must subdelegate blocks of address space to its NLA and SLA

customers. In this case, NLA I is assigned 3FFE:2900:1::/48, and NLA II

is assigned 3FFE:2900:2::/48. These NLAs then delegate blocks to their

customers out of this block. 

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