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

IPv6 Address Space Allocation 1 2 3 4 5 IPv6 Interface Identifiers and Physical Address Mapping

It is anticipated that unicast addressing will be used for the vast majority of Internet traffic under IPv6, just as is the case for older one, IPv4. It is for this reason that the largest of the assigned blocks of the IPv6 address space is dedicated to unicast addressing. A full 1/8th slice of the enormous IPv6 address “pie” is assigned to unicast addresses, which are indicated by a “001” in the first three bits of the address. The question then is: how do we use the remaining 125 bits in our spacious IP addresses?

When IPv4 was first created, the Internet was rather small, and the model for allocating address blocks was based on a central coordinator: IANA. Everyone who wanted address blocks would go straight the central authority. As the Internet grew, this model became impractical. Today, IPv4's classless addressing scheme allows variable-length network IDs and hierarchical assignment of address blocks. Big ISPs get large blocks from the central authority and then subdivide them and allocate them to their customers, and so on. This is managed by today's Internet providers, but there is nothing in the address space that helps manage the allocation process. In turn, each organization has the ability to further subdivide their address allocation to suit their internal requirements.

The designers of IPv6 had the benefit of this experience and realized there would be tremendous advantages to designing the unicast address structure to reflect the overall topology of the Internet. These include:

• Easier allocation of address blocks at various levels of the Internet topological hierarchy.
  
  
• IP network addresses that automatically reflect the hierarchy by which routers move information across the Internet, allowing routes to be easily aggregated for more efficient routing.
  
  
• Flexibility for organizations like ISPs to subdivide their address blocks for customers.
  
  
• Flexibility for end-user organizations to subdivide their address blocks to match internal networks, much as subnetting did in IPv4.
  
  
• Greater “meaning” to IP addresses. Instead of just being a string of 128 bits with no structure, it would become possible to look at an address and know certain things about it.

IPv6 Address Space Allocation 1 2 3 4 5 IPv6 Interface Identifiers and Physical Address Mapping