128-bit length
The length of network addresses emphasize a most important change when moving from IPv4 to IPv6. IPv6 addresses are 128 bits long (as defined by RFC 4291), whereas IPv4 addresses are 32 bits; where the IPv4 address space contains roughly 4 billion addresses, IPv6 has enough room for 3.4×1038 unique addresses.
IPv6 addresses are typically composed of two logical parts: a 64-bit (sub-) network prefix, and a 64-bit host part, which is either automatically generated from the interface's MAC address or assigned sequentially. Because the globally unique MAC addresses offer an opportunity to track user equipment, and so users, across time and IPv6 address changes, RFC 3041 was developed to reduce the prospect of user identity being permanently tied to an IPv6 address, thus restoring some of the possibilities of anonymity existing at IPv4. RFC 3041 specifies a mechanism by which time-varying random bit strings can be used as interface circuit identifiers, replacing unchanging and traceable MAC addresses.
Notation
IPv6 addresses are normally written as eight groups of four hexadecimal digits, where each group is separated by a colon (:). For example,
2001:0db8:85a3:0000:0000:8a2e:0370:7334
is a valid IPv6 address. To shorten the writing and presentation of addresses, several simplifications to the notation are permitted.
Any leading zeros in a group may be omitted; thus, the given example becomes
2001:db8:85a3:0:0:8a2e:370:7334
One or any number of consecutive groups of 0 value may be replaced with two colons (::):
2001:db8:85a3::8a2e:370:7334
This substitution with double-colon may be performed only once in an address, because multiple occurrences would lead to ambiguity. For example, the illegal address notation 2001::FFD3::57ab, could represent 2001:0:0:0:0:FFD3:0:57ab, 2001:0:0:0:FFD3:0:0:57ab, 2001:0:0:FFD3:0:0:0:57ab, and 2001:0:FFD3:0:0:0:0:57ab.
Accordingly, the localhost (loopback) address, fully written as 0000:0000:0000:0000:0000:0000:0000:0001, may be reduced to ::1 and the undetermined IPv6 address (zero value), i.e., all bits are zero, is simply ::.
For example, the addresses below are all valid and equivalent:
2001:0db8:0000:0000:0000:0000:1428:57ab
2001:0db8:0000:0000:0000::1428:57ab
2001:0db8:0:0:0:0:1428:57ab
2001:0db8:0:0::1428:57ab
2001:0db8::1428:57ab
2001:db8::1428:57ab
The sequence of the last 4 bytes of the IPv6 address may optionally be written in dot-decimal notation, in the style of IPv4 addresses. This notation is convenient when working in a mixed (dual-stack) environment of IPv4 and IPv6 addresses, and IPv6 addresses are derived from IPv4 ones. The general form of the notation is x:x:x:x:x:x:d.d.d.d, where the x's are the 6 high-order groups of hexadecimal digits and the d's represent the decimal digit groups of the four low-order octets of the address. For example, ::ffff:12.34.56.78 is the same address as ::ffff:0c22:384e. Usage of this notation may not be widely supported.
RFC 4291 (IP Version 6 Addressing Architecture) provides additional information.
Prefix and network notation
An IPv6 network is a contiguous group of IPv6 addresses. The size of this block must be a power of 2, and the beginning of a block must be aligned on a bit boundary of the address space. The leading set of bits of the addresses, which are identical for all hosts in a given network, are called the network's address prefix.
Networks are written in CIDR notation: a network is denoted by the first address in the network and the size in bits of the prefix, separated by a slash "/". For example, the network written 2001:0db8:1234::/48 starts at address 2001:0db8:1234:0000:0000:0000:0000:0000 and ends at 2001:0db8:1234:ffff:ffff:ffff:ffff:ffff.
Network addresses should not be confused with the notation used for interface addresses under some operating systems. Under such systems, an interface address is written by combining the address of the interface with the prefix length of the network it is connected to. For example, an interface with address 2001:db8:a::123 connected to a /64 subnet is written 2001:db8:a::123/64.
IPv6 address types
IPv6 addresses are classified into three types:
· Unicast addresses. A unicast address identifies a single network interface. The protocol delivers packets sent to a unicast address to that specific interface. Unicast IPv6 addresses can have a scope which is reflected in more specific address names: global unicast address, link-local address, and unique local unicast address.
· Anycast addresses. An anycast address is assigned to a group of interfaces, usually belonging to different nodes. A packet sent to an anycast address is delivered to just one of the member interfaces, typically the “nearest” according to the routing protocol’s choice of distance. Anycast addresses cannot be identified easily: they have the structure of normal unicast addresses, and differ only by being injected into the routing protocol at multiple points in the network.
· Multicast addresses. A multicast address is also assigned to a set of interfaces that typically belong to different nodes. A packet that is sent to a multicast address is delivered to all interfaces identified by that address. Multicast addresses begin with an octet of one (1) bits, i.e., they have prefix FF00::/8. The four least-significant bits of the second address octet identify the address scope, i.e. the span over which the multicast address is propagated.
Commonly implemented scopes are node-local (0x1), link-local (0x2), site-local (0x5), organization-local (0x8), and global (0xE). The least-significant 112 bits of a multicast address form the multicast group identifier. Only the low-order 32 bits of the group ID are commonly used, because of traditional methods of forming 32 bit identifiers from Ethernet addresses. Defined group IDs are 0x1 for all-nodes multicast addressing and 0x2 for all-routers multicast addressing.
Another group of multicast addresses are solicited-node multicast addresses which are formed with the prefix FF02::1:FF00:0/104, and where the rest of the group ID (least significant 24 bits) is filled from the interface's unicast or anycast address. These addresses allow link-layer address resolution via Neighbor Discovery Protocol (NDP) on the link without disturbing all nodes on the local network.
Special Address
IANA maintains the official list of the IPv6 address space. Global unicast assignments can be found at the various RIR's or at the GRH DFP pages.
There are a number of addresses with special meaning in IPv6:
Unspecified address
::/128 — the address with all zero bits is called the unspecified address. This address must never be assigned to an interface and is to be used only in software before the application has learned its host's source address appropriate for a pending connection. Routers must not forward packets with the unspecified address.
Link local addresses
::1/128 — the loopback address is a unicast localhost address. If an application in a host sends packets to this address, the IPv6 stack will loop these packets back on the same virtual interface (corresponding to 127.0.0.1 in IPv4).
fe80::/10 — The link-local prefix specifies that the address is only valid in the scope of a given local link. This is analogous to the Autoconfiguration IP addresses 169.254.0.0/16 in IPv4.
Unique local addresses
fc00::/7 — unique local addresses (ULA) are routable only within a set of cooperating sites. They were defined in RFC 4193 as a replacement for site-local addresses (see below). The addresses include a 40-bit pseudorandom number in the routing prefix that intends to minimize the risk of conflicts if sites merge or packets are misrouted into the Internet. Despite the restricted, local usage of these addresses, they have a global address scope. This is a departure from the site-local address definition that unique local addresses replaced.
Multicast addresses
ff00::/8 — The multicast prefix designates multicast addresses[14] as defined in "IP Version 6 Addressing Architecture" (RFC 4291). Some of these have been assigned to specific protocols, for example ff0X::101 will reach all local NTP servers (RFC 2375).
Solicited-node multicast addresses
ff02::1:FFXX:XXXX — XX:XXXX are the 3 low order octets of the corresponding unicast or anycast address.
IPv4 transition
::ffff:0:0/96 — this prefix is used for IPv4 mapped addresses (see Transition mechanisms below).
2001::/32 — Used for Teredo tunneling.
2002::/16 — this prefix is used for 6to4 addressing.
ORCHID
2001:10::/28 — ORCHID (Overlay Routable Cryptographic Hash Identifiers) as per (RFC 4843). These are non-routed IPv6 addresses used for Cryptographic Hash Identifiers.
Documentation
2001:db8::/32 — this prefix is used in documentation (RFC 3849). The addresses should be used anywhere an example IPv6 address is given, or model networking scenarios are described.
deprecated, or obsolete designations
::/96 — the zero prefix was used for IPv4-compatible addresses; it is now obsolete.
fec0::/10 — The site-local prefix specifies that the address is valid only inside the local organization. Its use has been deprecated in September 2004 by RFC 3879 and new systems must not support this special type of address.
Link-local addresses and zone indices
All interfaces have an associated link-local address, that is only guaranteed to be unique on the attached link. Link local addresses are created in the fe80::/10 address space.
Because link-local addresses have a common prefix, normal routing procedures cannot be used to choose the outgoing interface when sending packets to a link-local destination. A special identifier, known as a zone index, is needed to provide the additional information; in the case of link-local addresses, zone indices correspond to interface identifiers.
When an address is written textually, the zone index is appended to the address, separated by a percent sign "%". The actual syntax of zone indices depends on the operating system:
· the Microsoft Windows IPv6 stack uses numeric zone indexes, e.g., fe80::3%1. The index is determined by the interface number.
· Some Unix-like systems (e.g., BSD and Linux) use the interface name as a zone index: fe80::3%eth0.
Zone index notations cause syntax conflicts when used in Uniform Resource Identifiers (URI), as the '%' character also designates percent-encoding.
Relatively few IPv6-capable applications understand address scope syntax at the user level, thus rendering link-local addressing inappropriate for many user applications. However, link-local addresses are not intended for most of such application usage and their primary benefit is in low-level network management functions, for example for logging into a router that for some reason has become unreachable.
Literal IPv6 addresses in network resource identifiers
Since an IPv6 address contains colon (":") characters, network administrators must take care to avoid conflicts with other syntactic meanings of the colon in network resource labels. In IPv4 the colon is used to separate an IP address from a transport protocol port number. This usage has been extended to IPv6, however, when a port is specified in an address string, the proper IPv6 address must be enclosed in square brackets ("[", "]"). This convention is used in other more complex identifiers.
Example: In a URL the IPv6-Address is enclosed in brackets, e.g.,
http://[2001:0db8:85a3:08d3:1319:8a2e:0370:7348]/.
If the URL also contains a port number the notation is:
https://[2001:0db8:85a3:08d3:1319:8a2e:0370:7344]:443/
This is not only useful but mandated when using shortform:
https://[2001:db8::1428:57ab]:443/
Additional information can be found in "RFC 2732 - Format for Literal IPv6 Addresses in URL's" and "RFC 3986 - Uniform Resource Identifier (URI): Generic Syntax."
In Microsoft Windows operating systems, IP addresses were also allowed in Uniform Naming Convention (UNC) path names. Since the colon is an illegal character in a UNC path name, the use of IPv6 addresses is also illegal in UNC names. For this reason, Microsoft has registered a second-level Internet domain, ipv6-literal.net, as a means to facilitate symbolic substitution. IPv6 addresses may be transcribed in the following fashion:
2001:0db8:85a3:08d3:1319:8a2e:0370:7348
is written as
2001-db8-85a3-8d3-1319-8a2e-370-7348.ipv6-literal.net
This notation is automatically resolved by Microsoft software without DNS queries to any name servers. If the IPv6 address contains a zone index, it is appended to the address portion after an 's' character:
fe80--1s4.ipv6-literal.net.
Source:
http://en.wikipedia.org/wiki/IPv6
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