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IPv6 Addressing

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4/8/2010

IPv6 not only provides more IP addresses than its predecessor, IPv4, but also has much larger addresses. The following table shows the different address spaces provided by IPv6 and IPv4.

IP version Size of address space

IPv6

128 bits, which allows for 2128 or 340,282,366,920,938,463,463,374,607,431,768,211,456 (3.4 × 1038) possible addresses.

IPv4

32 bits, which allows for 232 or 4,294,967,296 possible addresses.

The relatively large size of the IPv6 address is designed to be subdivided into hierarchical routing domains that reflect the topology of the modern-day Internet. The use of 128 bits provides multiple levels of hierarchy and flexibility in designing hierarchical addressing and routing. The Ipv4-based Internet currently lacks this flexibility.

The architecture of IPv6 addressing is described in RFC 2373.

Every IPv6 address has a reachability scope. IPv6 interfaces can have multiple addresses that have different reachability scopes. For example, a node may have a link-local address and a global address.

The following table shows the address and associated reachability scopes.

Address scope Reachability scope Description

Node-local

Same node

Node-local addresses are used to send packets to the same node.

The following list shows the types of node-local addresses:

  • Loopback address
  • Node-local multicast address

The loopback address is equivalent to the IPv4 loopback address. Packets addressed to the loopback address are never sent on a link or forwarded by an IPv6 router.

Link-local

Local link

Link-local addresses are used to communicate between hosts on the link. Link-local addresses are always configured automatically.

The following list shows the types of link-local addresses:

  • Unspecified address
  • Link-local Unicast address
  • Link-local Multicast address

The unspecified address, indicates the absence of an address, and is typically used as a source address for packets that are attempting to verify the uniqueness of a tentative address. It is equivalent to the IPv4 unspecified address. The unspecified address is never assigned to an interface or used as a destination address.

Global

The IPv6 Internet

Global addresses, also known as aggregatable global unicast addresses, are globally routable and reachable on the IPv6 portion of the Internet. They are equivalent to public IPv4 addresses. Global addresses are configured by router advertisement.

The following list shows the types of global addresses:

  • Global Unicast address
  • Other scope Multicast address

Global addresses are designed to be aggregated or summarized to produce an efficient, hierarchical addressing and routing structure.

When you specify a link-local address, you should also specify a scope ID, which further defines the reachability scope for non-global addresses. For information about scope ID, see IPv6 Addresses.

Like the IPv4 address space, the IPv6 address space is divided according to the value of high order bits in the address. The high order bits and their fixed values are known as a Format Prefix.

The following table shows the IPv6 address space allocation by Format Prefixes.

Allocation Format Prefix Fraction of the address space

Reserved

0000 0000

1/256

Reserved for NSAP allocation

0000 001

1/128

Aggregatable global unicast addresses

001

1/8

Link-local unicast addresses

1111 1110 10

1/1024

Multicast addresses

1111 1111

1/256

The remainder of the IPv6 address space is unassigned.

The current set of unicast addresses that can be used with IPv6 nodes consists of aggregatable global unicast addresses and link-local unicast addresses. These represent only 15 percent of the entire IPv6 address space. For more information, see Unicast IPv6 Addresses.

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