tems. Large central office phone switches, for example, need only a

three digit national area code prefix compared to the current IPv4 sys 

tem, which uses a somewhat haphazard form of address hierarchy to

move traffic between networks attached to the Internet backbone. Fig 

ure 37 shows a globally routable unicast prefix divided into six sec 

tions.

F I G U R E   3 7 .   G l o b a l l y   R o u t a b l e   I P v 6   U n i c a s t   A d d re s s

A rc h i t e c t u re

The fields in the aggregatable global unicast address are:

  FP   Indicates the Format Prefix (FP). The Format Prefix for globally

routable unicast prefixes will always have the same three bits in the

initial deployment of IPv6. These first three bits indicate to any rout 

ing entity on the Internet that this address is a globally routable uni 

cast address. The FP will be unique to each type of IPv6 address,

making it easier for routing entities to discern packet types and

process them according to the rules that apply to the respective

packet type. The FP serves as a delimiter, enabling a routing device to

make a quick decision as to how to handle the incoming packet. This

addressing scheme allows the rest of the packet to be used for other

valuable information. 

  TLA ID   Indicates the Top Level Aggregator (TLA). TLAs are admin 

istered by IANA and allocated to local Internet registries that, in turn,

allocate individual TLAs to large Internet service providers (ISPs). A

13 bit field allows up to 8,192 TLAs. Routers in the highest level of

the IPv6 Internet routing hierarchy (called default free routers) do not

have a default route only routes with 16 bit prefixes corresponding to

the allocated TLAs.

  Res   Indicates bits reserved for future use in expanding the size of

either the TLA ID or the NLA ID. The size of this field is 8 bits.

  NLA ID    Indicates the Next Level Aggregator (NLA). The NLA ID is

used to identify a specific customer site. The size of this field is 24

bits. The NLA ID allows an ISP to create multiple levels of addressing

hierarchy within a network to organize addressing and routing for

downstream ISPs and to identify sites. The structure of the ISP's net 

work is transparent to the default free routers.

U N D E R S TA N D I N G   I P   A D D R E S S I N G

4 6