Chapter 1 IPv6 Address Configuration
that provide public network access) to control the top-level aggregation for
address space arrangement.
Such institutions can divide this 24-bit field for use in accordance with their own
addressing hierarchical structures. In this way, an entity can divide two bits of
address space into four internal top-level routes, and allocate the other 22 bits of
address space to other entities (for example, a small local ISP).
When these entities obtain enough address space, they can subdivide the
obtained space in the same way as mentioned above.
à
SLA ID
This is the site aggregation identifier and is used by some institutions to
arrange their internal network structures. Each institution can create its own
internal hierarchical network structure in the same way as that of IPv4.
When the 16-bit field is dedicated to the plane address space, there are
at most 65,535 different subnets available. If the first eight bits are used
for the internal advanced routing of this institution, then there will be 255
advanced subnets available, and each advanced subnet can have up to 255
sub-subnets.
à
Interface identifier
This is a 64-bit field, containing 64-bit values of the IEEE EUI-64 interface
identifier.
l
Special Address & Reserved Address
In the first 1/256 IPv6 address space, the first 8 bits 0000 0000 of all the addresses
are reserved. Most of the vacant address spaces are used for special addresses,
including:
à
Undesignated address
This is an all-zero address and is used if no valid address is available. For
example, if a host does not obtain an IPv6 address upon its initial startup
from the network, it can use this address. That is, it can specify this address
for the source address of the IPv6 packet when it sends out a request for
configuration information. This address can be expressed as 0:0:0:0:0:0:0:0,
or expressed as ::.
à
Loopback address
In IPv4, the loopback address is defined as 127.0.0.1. Any packet that sends
a loopback address must be sent to a network interface through protocol
stack, instead of being sent to the network link. The network interface itself
shall accept these packets in the same way as it receives packets from
external nodes, and transmits them back to the protocol stack.
The loopback function is used for software test and configuration. Except
the lowest bit, all the other bits of an IPv6 loopback address are 0, that is, a
loopback address can be expressed as 0:0:0:0:0:0:0:1 or ::1.
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