Answer: Unicast, Anycast and Multicast.

There are three types of IPv6 addresses, unicast, multicast, and anycast addresses.

The unicast and multicast addresses are similar to the IPv4 versions. However, IPv4 broadcast address is no longer supported and is replaced with a new type of address called anycast.

Unicast

Unicast is an identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address. A node can have more than one IPv6 network interface. Each separate interface must have its own unicast address associated with it. Contained in the 128-bit field is an address that identifies one interface.

64 bits

TLA ID

RES

NLA ID

SLA ID

Interface ID

· FP. The format prefix is the three-bit prefix to the IPv6 address that identifies where it belongs in the IPv6 address space.

· TLA ID. The top-level aggregation identifier contains the highest-level routing information of the address. This refers to the grossest level of routing information in the internetwork, and as currently defined (at 13 bits) there can be no more then 8192 different top-level routes.

· RES. The next eight bits are reserved for future use.

· NLA ID. The next-level aggregation identifier is 24 bits long, and it is meant to be used by organizations that control top-level aggregation Ids to organize that address space.

· SLA ID. The site-level aggregation identifier is the address space given to organizations for their internal network structure. With 16 bits available, each organization can create its own internal hierarchical network structure using subnets in the same way they are used in IPv4. As many as 65,535 different subnets are available using all 16 bits as a flat address space. Using the first eight bits for higher-level routing within the organization would allow 255 high-level subnets, each of which has as many as 255 sub-subnets.

· Interface ID. This 64-bit field contains a 64-bit value based on the IEEE EUI-64 interface ID

Multicast

Multicast is an identifier for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to all interfaces identified by that address. As soon as the first few bytes of a packet are received the node checks to see if the destination address is at the beginning of the transmission. If the destination address is the same as the node's interface address then the node will pick up the rest of the transmission. This makes it relatively simple for a node to pick up on broadcast and multicast transmissions. If a broadcast is sent then the node will listen. For multicasts, the node subscribes to a multicast address, and if it senses that the destination address is a multicast address, it must determine if it's a multicast address to which the node is then subscribed. When a node subscribes to a multicast address, it announces that it wants to be a member and any local routers will subscribe on behalf of that node. When a transmission is sent to that multicast address from another node that is on the same network, the IP multicast packet is encapsulated into a link layer multicast data transmission unit. The IPv6 solution to the broadcast problem is to use an "all nodes" multicast addresses to replace those broadcasts that are absolutely necessary, while resorting to more limited multicast addresses for other situations in which broadcasts were previously used. Below is an IPv6 multicast address format, from RFC 2373.

112 bits

11111111

Flags

Scope

Group ID

The first octet, which is all ones, identifies the address as a multicast address. Multicast addresses include a full 1/256th of the IPv6 address space, as shown above. The rest of the multicast address consists of three fields:

· Flags. This is actually a set of four single-bit flags. Only the fourth flag is currently assigned, and it represents whether or not the address is a well-know multicast address that has been assigned by the Internet numbering authority or is a temporary multicast address. If this flag is set to zero, it means the address is well-known; being set to one signifies a transient address. The other three flags are currently being reserved for future use.

· Scope. This four-bit field contains a value that indicates what the scope of the multicast group is. Whether the multicast group can include only nodes on the same local network, same site, same organization, or anywhere within the IPv6 global address space. Possible values range from 0 to 15 (hexadecimal) in table below.

Hex

Decimal

Value

reserved

node-local scope

link-local scope

(unassigned)

site-local scope

(unassigned)

organization-local scope

(unassigned)

global scope

reserve

· Group ID. This 112-bit field identifies the multicast group. The same group ID can represent different groups, depending on whether the address is transient or well known, and also depending on the scope of the address. Permanent multicast addresses use assigned group ID's with special meaning, and the membership in such groups will depend both on the group ID and on the scope.

Anycast

Anycast is an identifier for a set of interfaces (typically belonging to different nodes). A packet sent to an anycast address is delivered to one of the interfaces identified by that address (the "nearest" one, according to the routing protocols' measure of distance). All nodes that are members of a multicast address expect to receive all packets sent to that address. A router that connects five different local Ethernet networks will forward a copy of a multicast packet to each of those networks. Anycast is the same because multiple nodes may be sharing the anycast address, like a multicast address but different since only one of those nodes can expect to receive a datagram sent to the anycast address.

Address Format

IPv6 addresses are now four times as long as an IPv4 address. The IPv4 address is represented as X.X.X.X, where the "X" is any number from 0-255. IPv6 address on the other hand is in the form X:X:X:X:X:X:X:X, where X refers to a four-digit hexadecimal integer (16 bits). For example, a few valid IPv6 addresses are as follows:

CFAE:3290:ABCD:1234:CEAF:5678:9012:AAAA

ABC3:0000:0000:0003:ABCD:0123:FFFF:ABCD

The above IPv6 address could also be represented as

ABC3::3:ABCD:123:FFFF:ABCD

Note that the integers are hexadecimal integers, so the letters A through F represent the digits 10 through 15. Each integer must be included, but leading zeros are not required. In addition, a double colon (::) can be used once in an address to replace multiple fields of zeros. For example:

1000:0:0:0:0:0:0:1

could be represented as

1000::1

The double colon means that the address should be expanded out to a full 128-bit address. This method replaces zeros only when they fill a complete 16-bit group, and the double colon can be used only once in any given address.

IPv6 Headers

The new IPv6 header structure has a header boundary at 64-bits and has only 40 bytes, where 32 of them are used for IPv6 addresses and the remaining 8 bytes by 6 additional fields. Whereas IPv4 headers are terminated on a 32-bit boundary and consist of 24 bytes, where 8 of them are used for IPv4 address and the remaining 16 bytes by 12 additional fields. IPv6 headers do not contain any optional elements. If additional functions are need IPv6 uses extension headers. This makes the new IPv6 header much simpler then its predecessor. Below is a side-by-side comparison of the IPv4 and IPv6 header.

Version

Header Length

Service Type

Total Length

Identification

Flags

Fragment Offset

Time to Live

Protocol

Header Checksum

Source IP Address

Destination IP Address

Options

PAD

IPv4 Headers

Version

Priority

Flow Label

Payload Length

Next Header

Hop Limit

Source Address

Destination Address

IPv6 Headers

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