IPv6 has unicast and multicast addresses like IPv4. There is, however, no distinct concept of a broadcast address in IPv6. A new type of address, the anycast address, has been added to allow a message to be sent to any one member of a group of devices.
A unicast address identifies a single network interface. The Internet Protocol delivers packets sent to a unicast address to that specific interface.
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 host, according to the routing protocol's definition of distance. Anycast addresses cannot be identified easily, they have the same format as unicast addresses, and differ only by their presence in the network at multiple points. Almost any unicast address can be employed as an anycast address.
Anycast is a network addressing and routing methodology in which a single destination address has multiple routing paths to two or more endpoint destinations. Routers will select the desired path on the basis of number of hops, distance, lowest cost, latency measurements or based on the least congested route. Anycast networks are widely used for CDN products to bring their content closer to the end user.
Content Delivery Network or Content Distribution Network (CDN) is a geographically distributed network of proxy servers and their data centers. The goal is to distribute service spatially relative to end-users to provide high availability and high performance. CDNs serve a large portion of the Internet content today, including web objects (text, graphics and scripts), downloadable objects (media files, software, documents), applications (e-commerce, portals), live streaming media, on-demand streaming media, and social media sites.
A multicast address is also used by multiple hosts, which acquire the multicast address destination by participating in the multicast distribution protocol among the network routers. A packet that is sent to a multicast address is delivered to all interfaces that have joined the corresponding multicast group.
An IPv6 address is 128 bits in length and consists of eight, 16-bit fields, with each field bounded by a colon. Each field must contain a hexadecimal number.
Unicast and Anycast Address Format
Unicast and anycast addresses are typically composed of two logical parts : a 64-bit network prefix used for routing, and a 64-bit interface identifier used to identify a host's network interface.
General Unicast Address Format
|Fields||Routing Prefix||Subnet ID||Interface Identifier|
|Bits||48 (or more)||16 (or fewer)||64|
The network prefix (the routing prefix combined with the subnet id) is contained in the most significant 64 bits of the address. The size of the routing prefix may vary; a larger prefix size means a smaller subnet id size. The bits of the subnet id(entifier) field are available to the network administrator to define subnets within the given network. The 64-bit interface identifier is either automatically generated from the interface's MAC address using the modified EUI-64 format, obtained from a DHCPv6 server, automatically established randomly, or assigned manually.
Link-Local Address Format
A link-local address is also based on the interface identifier, but uses a different format for the network prefix.
The prefix field contains the binary value 1111111010. The 54 zeroes that follow make the total network prefix the same for all link-local addresses (fe80::/64 link-local address prefix), rendering them non-routable.
Multicast Address Format
Multicast addresses are formed according to several specific formatting rules, depending on the application.
The prefix holds the binary value 11111111 for any multicast address. Currently, 3 of the 4 flag bits in the flg field are defined; the most-significant flag bit is reserved for future use.
Multicast Address Flags
|Bit No.||Flag||Meaning When 0||Meaning When 1|
|9||R (Rendezvous)||Rendezvous point not embedded||Rendezvous point embedded|
|10||P (Prefix)||Without prefix information||Address based on network prefix|
|11||T (Transient)||Well-known multicast address||Dynamically assigned multicast address|
A 64-bit interface identifier is most commonly derived from its 48-bit MAC address. A MAC-Address 00-1F-29-00-0B-18 is turned into a 64-bit EUI-64 by inserting FF-FE in the middle : 00-1F-29-FF-FE-00-0B-18.
To convert EUI-64 ID into IPv6 Interface Identifier, the most significant 7th bit of EUI-64 ID is complemented.
After Complemented of 7th bit
Representation of IPv6 Address
An IPv6 address is represented as eight groups of four hexadecimal digits, each group representing 16 bits (two octets, a group sometimes also called a hextet). The groups are separated by colons (:). An example of an IPv6 address is :
The hexadecimal digits are case-insensitive, but IETF recommendations suggest the use of lower case letters. The full representation of eight 4-digit groups may be simplified by several techniques, eliminating parts of the representation.
Leading zeroes in a group may be omitted, but each group must retain at least one hexadecimal digit. Thus, the example address may be written as :
One or more consecutive groups containing zeros only may be replaced with a single empty group, using two consecutive colons (::).
The localhost (loopback) address, 0:0:0:0:0:0:0:1, and the IPv6 unspecified address, 0:0:0:0:0:0:0:0, are reduced to ::1 and ::, respectively.
(::) is not used to shorten just a single 0 field. For example, 2001:db8:0:0:0:0:2:1 is shortened to 2001:db8::2:1, but 2001:db8:0000:1:1:1:1:1 is rendered as 2001:db8:0:1:1:1:1:1.
The longest sequence of consecutive all-zero fields is replaced by double-colon. If there are multiple longest runs of all-zero fields, then it is the leftmost that is compressed. For example, 2001:db8:0:0:1:0:0:1 is rendered as 2001:db8::1:0:0:1 rather than as 2001:db8:0:0:1::1 or 2001:db8::1::1.
IPv6 Mixed Notation
During the transition of the Internet from IPv4 to IPv6, it is typical to operate in a mixed addressing environment. For such use cases, a special notation has been introduced, which expresses IPv4-mapped and IPv4-compatible IPv6 addresses by writing the least-significant 32 bits of an address in the familiar IPv4 dot-decimal notation, whereas the other 96 (most significant) bits are written in IPv6 format.
For example, The IPv4-mapped IPv6 address ::ffff:c000:0280 is written as ::ffff:192.0.2.128
and the IPv4-compatible IPv6 address ::c000:0280 is written as ::192.0.2.128
IPv6 Addresses in Network Resource Identifiers
Colon (:) characters in IPv6 addresses may conflict with the established syntax of resource identifiers, such as URIs and URLs. The colon has traditionally been used to terminate the host path before a port number. To alleviate this conflict, literal IPv6 addresses are enclosed in square brackets in such resource identifiers.
For example :
When the URL also contains a port number the notation is :
Special Address Blocks
|Prefix||Start Address||End Address||IPv4 Equivalent||Purpose|
|::ffff/96||::ffff:0:0||::ffff:ffff:ffff||Mapped all IPv4 Addresses||These addresses are used to embed IPv4 addresses in an IPv6 address.|
|Documentation - These addresses are used in examples and documentation. They should never be source or destination addresses.|
|2002::/16||2002::||2002:ffff:ffff:ffff:ffff:ffff:ffff:ffff||No Equivalent||6to4 addressing scheme - 6to4 is an Internet transition mechanism for migrating from Internet Protocol version 4 (IPv4) to version 6 (IPv6), a system that allows IPv6 packets to be transmitted over an IPv4 network (generally the IPv4 Internet) without the need to configure explicit tunnels.|
|Unique Local Addresses - These addresses are reserved for local use in home and enterprise environments and are not public address space.|
|fe80::/10||fe80::||febf:ffff:ffff:ffff:ffff:ffff:ffff:ffff||169.254.0.0/16||Link-Local Addresses - These addresses are used on a single link or a non-routed common access network, such as an Ethernet LAN. Routers must not forward IPv6 packets if the source or destination contains a link-local address.|
|ff00::/8||ff00::||ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff||220.127.116.11/4||Multicact Address - These addresses are used to identify multicast groups. They should only be used as destination addresses, never as source addresses.|
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