The IPv6 Address Space
The most obvious distinguishing feature of IPv6 is its use of much larger addresses. The size of an address in
IPv6 is 128 bits, which is four times the larger than an IPv4 address. A 32-bit address space allows for 232 or 4,294,967,296 possible addresses. A 128-bit address space allows for 2128 or 340,282,366,920,938,463,463,374,607,431,768,211,456 (or 3.4´1038 or 340 undecillion) possible addresses.
In the late 1970s when the IPv4 address space was designed, it was unimaginable that it could be exhausted.
However, due to changes in technology and an allocation practice that did not anticipate the recent explosion of hosts on the Internet, the IPv4 address space was consumed to the point that by 1992 it was clear a replacement would be necessary.
With IPv6, it is even harder to conceive that the IPv6 address space will be consumed. To help put this number in perspective, a 128-bit address space provides 655,570,793,348,866,943,898,599 (6.5´1023) addresses for every square meter of the Earth’s surface.
It is important to remember that the decision to make the IPv6 address 128 bits in length was not so that every square meter of the Earth could have 6.5´1023 addresses. Rather, the relatively large size of the IPv6 address is designed to be subdivided into hierarchical routing domains that reflect the topology of the modern-day Internet. The use of 128 bits allows for multiple levels of hierarchy and flexibility in designing hierarchical addressing and routing that is currently lacking on the IPv4-based Internet.
The IPv6 addressing architecture is described in RFC 4291.
IPv6 Address Syntax
IPv4 addresses are represented in dotted-decimal format. This 32-bit address is divided along 8-bit boundaries. Each set of 8 bits is converted to its decimal equivalent and separated by periods. For IPv6, the 128-bit address is divided along 16-bit boundaries, and each 16-bit block is converted to a 4-digit hexadecimal number and separated by colons. The resulting representation is called colon-hexadecimal.
The following is an IPv6 address in binary form:
0010000000000001000011011011100000000000000000000010111100111011 0000001010101010000000001111111111111110001010001001110001011010
The 128-bit address is divided along 16-bit boundaries:
0010000000000001 0000110110111000 0000000000000000 0010111100111011 0000001010101010 0000000011111111 1111111000101000 1001110001011010
Each 16-bit block is converted to hexadecimal and delimited with colons. The result is:
2001:0DB8:0000:2F3B:02AA:00FF:FE28:9C5A
IPv6 representation can be further simplified by removing the leading zeros within each 16-bit block. However, each block must have at least a single digit. With leading zero suppression, the address representation becomes:
2001:DB8:0:2F3B:2AA:FF:FE28:9C5A
Compressing Zeros
Some types of addresses contain long sequences of zeros. To further simplify the representation of IPv6 addresses, a contiguous sequence of 16-bit blocks set to 0 in the colon hexadecimal format can be compressed to “::”, known as double-colon.
For example, the link-local address of FE80:0:0:0:2AA:FF:FE9A:4CA2 can be compressed to FE80::2AA:FF:FE9A:4CA2. The multicast address FF02:0:0:0:0:0:0:2 can be compressed to FF02::2.
Zero compression can only be used to compress a single contiguous series of 16-bit blocks expressed in colon hexadecimal notation. You cannot use zero compression to include part of a 16-bit block. For example, you cannot express FF02:30:0:0:0:0:0:5 as FF02:3::5. The correct representation is FF02:30::5.
To determine how many 0 bits are represented by the “::”, you can count the number of blocks in the compressed address, subtract this number from 8, and then multiply the result by 16. For example, in the address FF02::2, there are two blocks (the “FF02” block and the “2” block.) The number of bits expressed by the “::” is 96 (96 = (8 – 2)´16).
Zero compression can only be used once in a given address. Otherwise, you could not determine the number of 0 bits represented by each instance of “::”.
IPv6 Prefixes
The prefix is the part of the address that indicates the bits that have fixed values or are the bits of the subnet prefix. Prefixes for IPv6 subnets, routes, and address ranges are expressed in the same way as Classless Inter-Domain Routing (CIDR) notation for IPv4. An IPv6 prefix is written in address/prefix-length notation.
For example, 21DA:D3::/48 and 21DA:D3:0:2F3B::/64 are IPv6 address prefixes.
Note IPv4 implementations commonly use a dotted decimal representation of the network prefix known as the subnet mask. A subnet mask is not used for IPv6. Only the prefix length notation is supported.
Types of IPv6 Addresses
There are three types of IPv6 addresses:
1. Unicast
A unicast address identifies a single interface within the scope of the type of unicast address. With the appropriate unicast routing topology, packets addressed to a unicast address are delivered to a single interface.
2. Multicast
A multicast address identifies multiple interfaces. With the appropriate multicast routing topology, packets addressed to a multicast address are delivered to all interfaces that are identified by the address. A multicast address is used for one-to-many communication, with delivery to multiple interfaces.
3. Anycast
An anycast address identifies multiple interfaces. With the appropriate routing topology, packets addressed to an anycast address are delivered to a single interface, the nearest interface that is identified by the address.
The “nearest” interface is defined as being closest in terms of routing distance. An anycast address is used for one-to-one-of-many communication, with delivery to a single interface.
In all cases, IPv6 addresses identify interfaces, not nodes. A node is identified by any unicast address assigned to one of its interfaces.
Note RFC 4291 does not define a broadcast address. All types of IPv4 broadcast addressing are performed in IPv6 using multicast addresses. For example, the subnet and limited broadcast addresses from IPv4 are replaced with the link-local scope all-nodes multicast address of FF02::1.
Links and Subnets
Similar to IPv4, an IPv6 subnet prefix is assigned to a single link. Multiple subnet prefixes can be assigned to the same link. This technique is called multinetting.
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