Addressing Architecture of IPv6
IPv6 has a large address space because of its 128-bit address scheme. As discussed in this chapter, several parts of this address space are used for the functions of the protocol itself such link-local, site-local, multicast address, multicast assigned address, solicited-node multicast address, loopback, unspecified, and IPv4-compatible IPv6 address. Although several parts of the 128-bit address are used, only a small percentage (less than 2%) of the whole space is reserved for those functions.
Table 2-26 presents an overview of spaces allocated compared to IPv6's entire addressing space. The first column, Prefix in Binary, represents the high-order 16-bit of each allocation. The character x means that these bits may have any binary value. The second column is the range in hexadecimal values for the allocation. The next two columns show the ratio and percentage per allocation compared to the whole IPv6 space. The last column describes the specific use of the allocation.
| Prefix in Binary (High-Order 16-Bit) | Range in Hexadecimal | Size (Ratio) | % | Description of the Allocation |
|---|---|---|---|---|
| 0000 0000 xxxx xxxx | 0000 to 00FF | 1/256 | 0.38% | Unspecified, loopback, IPv4-compatible address |
| 0000 0001 xxxx xxxx | 0100 to 01FF | 1/256 | 0.38% | Unassigned |
| 0000 001x xxxx xxxx | 0200 to 03FF | 1/128 | 0.77% | NSAP |
| 0000 010x xxxx xxxx | 0400 to 05FF | 1/128 | 0.77% | Unassigned |
| 0000 011x xxxx xxxx | 0600 to 07FF | 1/128 | 0.77% | Unassigned |
| 0000 1xxx xxxx xxxx | 0800 to 0FFF | 1/32 | 3.13% | Unassigned |
| 0001 xxxx xxxx xxxx | 1000 to 1FFF | 1/16 | 6.26% | Unassigned |
| 001x xxxx xxxx xxxx | 2000 to 3FFF | 1/8 | 12.5% | Aggregatable global unicast addresses (IANA) |
| 010x xxxx xxxx xxxx | 4000 to 5FFF | 1/8 | 12.5% | Unassigned |
| 011x xxxx xxxx xxxx | 6000 to 7FFF | 1/8 | 12.5% | Unassigned |
| 100x xxxx xxxx xxxx | 8000 to 9FFF | 1/8 | 12.5% | Unassigned |
| 101x xxxx xxxx xxxx | A000 to BFFF | 1/8 | 12.5% | Unassigned |
| 110x xxxx xxxx xxxx | C000 to DFFF | 1/8 | 12.5% | Unassigned |
| 1110 xxxx xxxx xxxx | E000 to EFFF | 1/16 | 6.26% | Unassigned |
| 1111 0xxx xxxx xxxx | F000 to F7FF | 1/32 | 3.13% | Unassigned |
| 1111 10xx xxxx xxxx | F800 to FBFF | 1/64 | 1.6% | Unassigned |
| 1111 110x xxxx xxxx | FC00 to FDFF | 1/128 | 0.77% | Unassigned |
| 1111 1110 0xxx xxxx | FE00 to FE7F | 1/512 | 0.2% | Unassigned |
| 1111 1110 10xx xxxx | FE80 to FEBF | 1/1024 | 0.1% | Link-local |
| 1111 1110 11xx xxxx | FEC0 to FEFF | 1/1024 | 0.1% | Site-local |
| 1111 1111 xxxx xxxx | FF00 to FFFF | 1/256 | 0.38% | Multicast |
Here are the highlights of the IPv6 address space allocations:
00::/8 or ::/8 is the range reserved for unspecified (::), loopback (::1), and IPv4-compatible addresses (::/96). This allocation uses about 0.38% (1/256) of the address space.
200::/7 is reserved for Network Service Access Point (NSAP) allocation, which uses 0.77% (1/128) of the space. There is no current use of this reserved space for NSAP. NSAP addresses are mainly used in ATM technologies. In the past, another address range was reserved for the IPX (Internetwork Packet Exchange) protocol. However, the reserved range for IPX was deprecated.
2000::/3 is the aggregatable global unicast address allocation, which uses 12.5% (1/8) of the address space. Aggregatable global unicast addresses are production IPv6 addresses for the IPv6 Internet. However, this range contains a total of 8192 /16 prefixes, and the IANA has started the assignment of public addresses only with prefixes 2001::/16, 2002::/16, and 3FFE::/16 (3 out of 8192). Note that one /16 prefix can handle several times the whole IPv4 Internet address space.
FE80::/10 is the link-local address space, which uses 0.1% (1/1024) of the whole space. Each network interface has one link-local address automatically assigned.
FEC0::/10 is the site-local address space, which uses 0.1% (1/1024) of the whole space. Site-local addresses can be used internally within any network.
FF00::/8 is the multicast address space, which uses 0.38% (1/256) of the space. Multicast addresses are used in the basic operation of the IPv6 protocol.
Less than 2% of the whole addressing is reserved or assigned for real use.
Table 2-26 demonstrates that IPv6's 128-bit address scheme provides enough addresses for the next decades.