IPv6 Addressing


IPv6 Address Representation
You know the 32-bit IPv4 address as a series of four 8-bit fields, separated by dots. However, larger 128-bit IPv6 addresses need a different representation because of their size. IPv6 addresses use colons to separate entries in a series of 16-bit hexadecimal.
Consider the address 2031:0000:130F:0000:0000:09C0:876A:130B. IPv6 does not require explicit address string notation.
Leading zeros in a field are optional. For example, the field 09C0 equals 9C0, and the field 0000 equals 0. So 2031:0000:130F:0000:0000:09C0:876A:130B can be written as 2031:0:130F:0000:0000:9C0:876A:130B.
Successive fields of zeros can be represented as two colons "::". However, this shorthand method can only be used once in an address. For example 2031:0:130F:0000:0000:9C0:876A:130B can be written as 2031:0:130F::9C0:876A:130B.
An unspecified address is written as "::" because it contains only zeros.
Using the "::" notation greatly reduces the size of most addresses as shown. An address parser identifies the number of missing zeros by separating any two parts of an address and entering 0s until the 128 bits are complete.

IPv6 Global Unicast Address
IPv6 has an address format that enables aggregation upward eventually to the ISP. Global unicast addresses typically consists of a 48-bit global routing prefix and a 16-bit subnet ID. Individual organizations can use a 16-bit subnet field to create their own local addressing hierarchy. This field allows an organization to use up to 65,535 individual subnets.
At the top of the figure, it can be seen how additional hierarchy is added to the 48-bit global routing prefix with the registry prefix, ISP Prefix, and site prefix.
The current global unicast address that is assigned by the IANA uses the range of addresses that start with binary value 001 (2000::/3), which is 1/8 of the total IPv6 address space and is the largest block of assigned addresses. The IANA is allocating the IPv6 address space in the ranges of 2001::/16 to the five RIR registries (ARIN, RIPE, APNIC, LACNIC, and AfriNIC).
For more information, refer to RFC 3587, IPv6 Global Unicast Address Format, which replaces RFC 2374.

R
eserved Addresses

The IETF reserves a portion of the IPv6 address space for various uses, both present and future. Reserved addresses represent 1/256th of the total IPv6 address space. Some of the other types of IPv6 addresses come from this block.

P
rivate Addresses

A block of IPv6 addresses is set aside for private addresses, just as is done in IPv4. These private addresses are local only to a particular link or site, and are therefore never routed outside of a particular company network. Private addresses have a first octet value of "FE" in hexadecimal notation, with the next hexadecimal digit being a value from 8 to F.
These addresses are further divided into two types, based upon their scope.
Site-local addresses, are addresses similar to the RFC 1918 Address Allocation for Private Internets in IPv4 today. The scope of these addresses is an entire site or organization. However, the use of site-local addresses is problematic and is being deprecated as of 2003 by RFC 3879. In hexadecimal, site-local addresses begin with "FE" and then "C" to "F" for the third hexadecimal digit. So, these addresses begin with "FEC", "FED", "FEE", or "FEF".
Link-local addresses, are new to the concept of addressing with IP in the Network layer. These addresses have a smaller scope than site-local addresses; they refer only to a particular physical link (physical network). Routers do not forward datagrams using link-local addresses at all, not even within the organization; they are only for local communication on a particular physical network segment. They are used for link communications such as automatic address configuration, neighbor discovery, and router discovery. Many IPv6 routing protocols also use link-local addresses. Link-local addresses begin with "FE" and then have a value from "8" to "B" for the third hexadecimal digit. So, these addresses start with "FE8", "FE9", "FEA", or "FEB".

Loopback Address
Just as in IPv4, a provision has been made for a special loopback IPv6 address for testing; datagrams sent to this address "loop back" to the sending device. However, in IPv6 there is just one address, not a whole block, for this function. The loopback address is 0:0:0:0:0:0:0:1, which is normally expressed using zero compression as "::1".

Unspecified Address
In IPv4, an IP address of all zeroes has a special meaning; it refers to the host itself, and is used when a device does not know its own address. In IPv6, this concept has been formalized, and the all-zeroes address (0:0:0:0:0:0:0:0) is named the "unspecified" address. It is typically used in the source field of a datagram that is sent by a device that seeks to have its IP address configured. You can apply address compression to this address; because the address is all zeroes, the address becomes just "::".

IPv6 Address Management
IPv6 addresses use interface identifiers to identify interfaces on a link. Think of them as the host portion of an IPv6 address. Interface identifiers are required to be unique on a specific link. Interface identifiers are always 64 bits and can be dynamically derived from a Layer 2 address (MAC).
You can assign an IPv6 address ID statically or dynamically:
Static assignment using a manual interface ID
Static assignment using an EUI-64 interface ID
Stateless autoconfiguration
DHCP for IPv6 (DHCPv6)

Manual Interface ID Assignment
One way to statically assign an IPv6 address to a device is to manually assign both the prefix (network) and interface ID (host) portion of the IPv6 address. To configure an IPv6 address on a Cisco router interface, use the ipv6 address ipv6-address/prefix-length command in interface configuration mode. The following example shows the assignment of an IPv6 address to the interface of a Cisco router:
RouterX(config-if)#ipv6 address 2001:DB8:2222:7272::72/64
EUI-64 Interface ID Assignment
Another way to assign an IPv6 address is to configure the prefix (network) portion of the IPv6 address and derive the interface ID (host) portion from the Layer 2 MAC address of the device, which is known as the EUI-64 interface ID.
The EUI-64 standard explains how to stretch IEEE 802 MAC addresses from 48 to 64 bits by inserting the 16-bit 0xFFFE in the middle at the 24th bit of the MAC address to create a 64-bit, unique interface identifier.
To configure an IPv6 address on a Cisco router interface and enable IPv6 processing using EUI-64 on that interface, use the ipv6 addressipv6-prefix/prefix-lengtheui-64 command in interface configuration mode. The following example shows the assignment of an EUI-64 address to the interface of a Cisco router:

RouterX(config-if)#ipv6 address 2001:DB8:2222:7272::/64 eui-64

Stateless Autoconfiguration
Autoconfiguration automatically configures the IPv6 address. In IPv6, it is assumed that non-PC devices, as well as computer terminals, will be connected to the network. The autoconfiguration mechanism was introduced to enable plug-and-play networking of these devices to help reduce administration overhead.
DHCPv6 (Stateful)
DHCPv6 enables DHCP servers to pass configuration parameters, such as IPv6 network addresses, to IPv6 nodes. It offers the capability of automatic allocation of reusable network addresses and additional configuration flexibility. This protocol is a stateful counterpart to IPv6 stateless address autoconfiguration (RFC 2462), and can be used separately or concurrently with IPv6 stateless address autoconfiguration to obtain configuration parameters.
For more information on IPv6 address assignment visit the following: http://www.netbsd.org/docs/network/ipv6/.

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