As defined first by RFC 1884 and later revised
by RFC 2373, IPv6 addresses are 128-bit identifiers for interfaces and
sets of interfaces, not nodes. Three general types of addresses exist:
- Unicast - An identifier for a
single interface. A packet sent to a unicast address is delivered to
the interface identified by that address.
- Anycast - An identifier for a set
of interfaces (typically belonging to different nodes). A packet sent
to an anycast address is delivered to the "nearest," or
first, interface in the anycast group.
- Multicast - An identifier for a
set of interfaces (typically belonging to different nodes). A packet
sent to a multicast address is delivered to all interfaces in the
multicast group.
To write 128-bit addresses so that they are
readable to human eyes, IPv6's architects abandoned dotted-decimal
notation in favor of a hexadecimal format. Therefore, IPv6 can be written as 32
hex digits, with colons separating the values of the eight 16-bit pieces
of the address, as shown in Figure .
Under current plans, IPv6 nodes that connect to the Internet will use what
is called an aggregatable global unicast address, which is the counterpart
to IPv4 global addresses that you are already familiar with. Like CIDR-enhanced
IPv4, aggregatable global unicast addresses rely on hierarchy to keep
Internet routing tables manageable. IPv6 global unicast addresses feature
three levels of hierarchy:
- Public topology - The collection
of providers that provide Internet connectivity
- Site topology - The level local
to an organization that does not provide connectivity to nodes outside
itself
- Interface identifier - The level
specific to a node's individual interface
This three-level hierarchy is reflected by
the structure of the aggregatable global unicast address, which includes
the following fields (see Figure ):
- FP field (3 bits) - The 3-bit
Format Prefix (FP) is used to identify the type of address (unicast,
multicast, and so on). The bits 001 identify aggregatable global
unicasts.
- TLA ID field (13 bits) - The
Top-Level Aggregation Identifier (TLA ID) field is used to identify
the authority responsible for the address at the highest level of the
routing hierarchy. Internet routers will necessarily maintain routes
to all TLA IDs. With 13 bits set aside, this field can represent up to
8,192 TLAs.
- Res field (8 bits) - IPv6
architect defined the reserved (Res) field so that the TLA or NLA IDs
could be expanded as future growth warrants. Currently, this field
must be set to zero.
- NLA ID field (24 bits) - The
Next-Level Aggregation Identifier (NLA ID) field is used to identify
ISPs. The field itself can be organized hierarchically to reflect a
hierarchy, or multitiered relationship, among providers.
- SLA ID field (16 bits) - The
Site-Level Aggregation Identifier (SLA ID) is used by an individual
organization to create its own local addressing hierarchy and to
identify subnets.
- Interface ID field (64 bits) - The
Interface ID field is used to identify individual interfaces on a link.
This field is analogous to the host portion of an IPv4 address, but it is
derived using the IEEE EUI-64 format, which, on LAN interfaces, adds a
16-bit field to the interface's MAC address.
In addition to the global unicast address space, IPv6 offers internal
network numbers, or "site local use" addresses, which are
analogous to RFC 1918 addresses. If a node is not addressed with a global
unicast address or an internal site local use address, it can be addressed
using a link local use address, which is specific a network segment.
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