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 Addressing Overview: Addressing Model and Address Types 1 2 3 IPv6 Address and Address Notation and Prefix Representation

Of all the changes introduced in IPv6, easily the most “celebrated” is the increase in the size of IP addresses, and as a result, the increase in the size of the address space as well. It's not surprising that these sizes were increased compared to IPv4—everyone has known for years that the IPv4 address space was too small to support the future of the Internet. What's remarkable is just how much the increase is, and what the implications are for how Internet addresses are used.

In IPv4, IP addresses are 32 bits long; these are usually grouped into four octets of eight bits each. The theoretical IPv4 address space is 2³², or 4,294,967,296 addresses. To increase this address space we simply increase the size of addresses; each extra bit we give to the address size doubles the address space. Based on this, some folks expected the IPv6 address size to increase from 32 to 48 bits, or perhaps 64 bits. Either of these numbers would have given a rather large number of addresses.

However, IPv6 addressing doesn't use either of these figures; instead, the IP address size jumps all the way to 128 bits, or sixteen 8-bit octets/bytes. This represents a truly remarkable increase in the address size, which surprised a lot of people.

IPv6 Addressing Overview: Addressing Model and Address Types 1 2 3 IPv6 Address and Address Notation and Prefix Representation