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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)
                     9  IPv6 Addressing

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IPv6 Address Space Allocation
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IPv6 Interface Identifiers and Physical Address Mapping
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IPv6 Global Unicast Address Format
(Page 2 of 5)

Generic Division of the Unicast Address Space

The most generic way of dividing up the 128 bits of the unicast address space is into three sections, as shown in Table 61.


Table 61: Generic IPv6 Global Unicast Address Format

Field Name

Size (bits)

Description

Prefix

“n”

Global Routing Prefix: The network ID or prefix of the address, used for routing.

Subnet ID

“m”

Subnet Identifier: A number that identifies a subnet within the site.

Interface ID

“128-n-m”

Interface ID: The unique identifier for a particular interface (host or other device). It is unique within the specific prefix and subnet.


The global routing prefix and subnet identifier represent the two basic levels at which addresses need to be hierarchically-constructed: global and site-specific. The routing prefix consists of a number of bits that can be further subdivided according to the needs of Internet Registries and Internet Service Providers, to reflect the topography of the Internet as a whole (we'll discuss this soon). The subnet ID gives a number of bits to site administrators for creating their own internal network structure.

IPv6 Implementation of the Unicast Address Space

In theory, any size for “n” and “m” could be used. The implementation chosen for IPv6, however, assigns 48 bits to the routing prefix and 16 bits to the subnet identifier. This means 64 bits are available for interface identifiers, which are constructed based on the IEEE “EUI-64” format, as described in the next topic. Thus, the overall IPv6 unicast address format is constructed as shown in Table 62 (illustrated in Figure 96.)


Table 62: IPv6 Global Unicast Address Format

Field Name

Size (bits)

Description

Prefix

48

Global Routing Prefix: The network ID or prefix of the address, used for routing. The first three bits are “001” to indicate a unicast address.

Subnet ID

16

Subnet Identifier: A number that identifies a subnet within the site.

Interface ID

64

Interface ID: The unique identifier for a particular interface (host or other device). It is unique within the specific prefix and subnet.



Figure 96: IPv6 Global Unicast Address Format

 


Key Concept: The part of the IPv6 address space set aside for unicast addresses is structured into an address format that uses the first 48 bits for the routing prefix (like a network ID), the next 16 bits for a subnet ID, and the final 64 bits for an interface ID (like a host ID).


Due to this structure, most end sites (regular companies and organizations, as opposed to Internet service providers) will be assigned IPv6 networks with a 48-bit prefix. In common parlance, these network identifiers have now come to be called 48s or /48s.

The 16 bits of subnet ID allow each site considerable flexibility in creating subnets that reflect the site's network structure. For example:

  • A smaller organization can just set all the bits in the Subnet ID to zero and have a “flat” internal structure.

  • A medium-sized organization could use all the bits in the Subnet ID to perform the equivalent of “straight” subnetting under IPv4, assigning a different Subnet ID to each subnet. There are 16 bits here, so this allows a whopping 65,536 subnets!

  • A larger organization can use the bits to create a multiple-level hierarchy of subnets, exactly like IPv4's Variable Length Subnet Masking (VLSM). For example, the company could use two bits to create four subnets. It could then take the next three bits to create eight sub-subnets in some or all of the four subnets. There would still be 11 more bits to create sub-sub-subnets and so forth.

Previous Topic/Section
IPv6 Address Space Allocation
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2
345
Next Page
IPv6 Interface Identifiers and Physical Address Mapping
Next Topic/Section

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