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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 and Address Notation and Prefix Representation 1 2 3 IPv6 Global Unicast Address Format

Looking at the IPv6 Address Space Plan As “Eight Eighths”

An easier way to make sense of this table is to consider the division of the IPv6 address space into eighths. Of these eight groups, one (001) has been reserved for unicast addresses; a second (000) has been used to carve out smaller reserved blocks, and a third (111) has been used for sub-blocks for local and multicast addresses. Five are completely unassigned.

You can see that the IPv6 designers have taken great care to allocate only the portion of these “eighths” of the address space they felt was needed for each type of address. For example, only a small portion of the part of the address space beginning “111” was used, with most of it left aside. In total, only 71/512ths of the address space is assigned right now, or about 14%. The other 86% is unassigned and kept aside for future use. (Bear in mind that even 1/1024th of the IPv6 address space is gargantuan—it represents trillions of trillions of addresses.)

Subsequent topics in this section provide more information on several of these address blocks. Note that the “0000 0000” reserved block is used for several special address types, including the loopback address, the “unspecified” address and IPv4 address embedding. The “1111 1111” format prefix identifies multicast addresses; this string is “FF” in hexadecimal, so any address beginning with “FF” is a multicast address in IPv6.

IPv6 Address and Address Notation and Prefix Representation 1 2 3 IPv6 Global Unicast Address Format