IPv6 Aggregatable global unicast address
An aggregatable global address is an IPv6 address from the
aggregatable global unicast prefix. The structure of aggregatable global
unicast addresses enables strict aggregation of routing prefixes that limits
the number of routing table entries in the global routing table. Aggregatable
global addresses are used on links that are aggregated upward through
organizations, and eventually to the Internet service providers (ISPs).
Aggregatable global IPv6 addresses are defined by a global routing
prefix, a subnet ID, and an interface ID. Except for addresses that start with
binary 000, all global unicast addresses have a 64-bit interface ID. The IPv6
global unicast address allocation uses the range of addresses that start with
binary value 001 (2000::/3). The figure below shows the structure of an
aggregatable global address.
|
Figure
1
|
Aggregatable
Global Address Format
|
Addresses with a prefix of 2000::/3 (001) through E000::/3 (111)
are required to have 64-bit interface identifiers in the extended universal
identifier (EUI)-64 format. The Internet Assigned Numbers Authority (IANA)
allocates the IPv6 address space in the range of 2000::/16 to regional
registries.
The aggregatable global address typically consists of a 48-bit
global routing prefix and a 16-bit subnet ID or Site-Level Aggregator (SLA). In
the IPv6 aggregatable global unicast address format document (RFC 2374), the
global routing prefix included two other hierarchically structured fields named
Top-Level Aggregator (TLA) and Next-Level Aggregator (NLA). The IETF decided to
remove the TLS and NLA fields from the RFCs because these fields are
policy-based. Some existing IPv6 networks deployed before the change might
still be using networks based on the older architecture.
A 16-bit subnet field called the subnet ID could be used by
individual organizations to create their own local addressing hierarchy and to
identify subnets. A subnet ID is similar to a subnet in IPv4, except that an
organization with an IPv6 subnet ID can support up to 65,535 individual
subnets.
An interface ID is used to identify interfaces on a link. The
interface ID must be unique to the link. It may also be unique over a broader
scope. In many cases, an interface ID will be the same as or based on the
link-layer address of an interface. Interface IDs used in aggregatable global
unicast and other IPv6 address types must be 64 bits long and constructed in
the modified EUI-64 format.
Interface IDs are constructed in the modified EUI-64 format in one
of the following ways:
- For all IEEE 802
interface types (for example, Ethernet, and FDDI interfaces), the first
three octets (24 bits) are taken from the Organizationally Unique
Identifier (OUI) of the 48-bit link-layer address (the Media Access
Control [MAC] address) of the interface, the fourth and fifth octets (16
bits) are a fixed hexadecimal value of FFFE, and the last three octets (24
bits) are taken from the last three octets of the MAC address. The
construction of the interface ID is completed by setting the
Universal/Local (U/L) bit--the seventh bit of the first octet--to a value
of 0 or 1. A value of 0 indicates a locally administered identifier; a
value of 1 indicates a globally unique IPv6 interface identifier.
- For other interface types
(for example, serial, loopback, ATM, Frame Relay, and tunnel interface
types--except tunnel interfaces used with IPv6 overlay tunnels), the
interface ID is constructed in the same way as the interface ID for IEEE
802 interface types; however, the first MAC address from the pool of MAC
addresses in the router is used to construct the identifier (because the
interface does not have a MAC address).
- For tunnel interface
types that are used with IPv6 overlay tunnels, the interface ID is the
IPv4 address assigned to the tunnel interface with all zeros in the
high-order 32 bits of the identifier.
|
Note |
For interfaces using Point-to-Point Protocol (PPP), given that
the interfaces at both ends of the connection might have the same MAC
address, the interface identifiers used at both ends of the connection are
negotiated (picked randomly and, if necessary, reconstructed) until both
identifiers are unique. The first MAC address in the router is used to
construct the identifier for interfaces using PPP.
|
If no IEEE 802 interface types are in the router, link-local IPv6
addresses are generated on the interfaces in the router in the following
sequence:
1.
The router is
queried for MAC addresses (from the pool of MAC addresses in the router).
2.
If no MAC
addresses are available in the router, the serial number of the router is used
to form the link-local addresses.
3.
If the serial number of the router cannot be
used to form the link-local addresses, the router uses a message digest
algorithm 5 (MD5) hash to determine the MAC address of the router from the
hostname of the router.
