IPv6 Address Types

Several IPv6 types of addressing schemes are specified in the RFC 2373 (IPv6 Addressing Architecture). IPv6 addresses are 128-bit identifiers for interfaces and sets of interfaces. There are various address formats for IPv6 defined by the RFC 2373. IPv6 addresses are broadly classified as follows:

The following table explains the three types of IPv6 address types: unicast, anycast, and multicast.

Table H-1 IPv6 Address Types

Unicast	An address for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address.
Anycast	An address for a set of interfaces. In most cases these interfaces belong to different nodes. A packet sent to an anycast address is delivered to one of these interfaces identified by the address. Since the standards for using anycast addresses is still evolving, they are not supported in HP-UX as of now.
Multicast	An address for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address will be delivered to all interfaces identified by that address.

Unlike IPv4, there are no broadcast addresses in IPv6 because their functions are superseded by multicast.

Textual Representation of IPv6 Addresses

There are three conventional forms for representing IPv6 addresses as text strings:

The first form is x:x:x:x:x:x:x:x, where x’s are the hexadecimal values of eight 16-bit pieces of the 128-bit address. Example:
2001:fecd:ba23:cd1f:dcb1:1010:9234:4088.
Some of the IPv6 addresses may contain a long strings of zero bits. In order to make it easy for representing such addresses textually a special syntax is available. The use of “::” indicates that there are multiple groups of 16-bits of zeros. The “::” can appear only once in an address and it can be used to compress the leading, trailing, or contiguous sixteen-bit zeroes in an address. Example:
fec0:1:0:0:0:0:0:1234 can be represented as fec0:1::1234.
When dealing with a mixed environment of IPv4 and IPv6 nodes there is an alternative form of IPv6 address that will be used. It is x:x:x:x:x:x:d.d.d.d, where 'x's are the hexadecimal values of higher order 96 bits of IPv6 address and the 'd's are the decimal values of the 32-bit lower order bits. Typically IPv4 Mapped IPv6 addresses and IPv4 Compatible IPv6 addresses will be represented in this notation. These addresses will be discussed in later sections.
Examples:
0:0:0:0:0:0:10.1.2.3
and
::10.11.3.123

IPv6 Address Prefix

IPv6 Address Prefix is similar to CIDR in IPv4 and is written in CIDR notation. An IPv6 address prefix is represented by the notation:

IPv6-address/prefix-length where ipv6-address is an IPv6 address in any notation listed above and prefix-length is a decimal value representing how many of the leftmost contiguous bits of the address comprise the prefix. Example:

fec0:0:0:1::1234/64

The first 64-bits of the address fec0:0:0:1 forms the address prefix. An address prefix is used in IPv6 addresses to denote how many bits in the IPv6 address represent the subnet.

Unicast Addresses

IPv6 unicast addresses are classified into different types. They are global aggregatable unicast address, site-local address and link-local address. Typically a unicast address is logically divided as follows:

Table H-2 Title not available (Unicast Addresses)

n bits	128-n bits
Subnet prefix	Interface ID

Interface identifiers in a IPv6 unicast address are used to identify the interfaces on a link. Interface identifiers are required to be unique on that link. The link is generally identified by the subnet prefix.

A unicast address is called an unspecified address if all the bits in the address are zero. Textually it is represented as “::”.

The unicast address ::1 or 0:0:0:0:0:0:0:1 is called the loopback address. It is used by a node to send packets to itself.

IPv4 and IPv6 Compatibility

There are a number of techniques for using IPv4 addresses within the framework of IPv6 addressing.

IPv4 Compatible IPv6 Addresses

The IPv6 transition mechanisms use a technique for tunneling IPv6 packets over the existing IPv4 infrastructure. IPv6 nodes that support such mechanisms use a special kind of IPv6 addresses that carry IPv4 addresses in their lower order 32-bits. These addresses are called IPv4 Compatible IPv6 addresses. They are represented as follows:

Table H-3 Title not available (IPv4 Compatible IPv6 Addresses)

80 bits	16 bits	32 bits
zeros	0000	IPv4 address

Example:

::192.168.0.1

IPv4 Mapped IPv6 Address

There is a special type of IPv6 address that holds an embedded IPv4 address. This address is used to represent the addresses of IPv4-only nodes as IPv6 addresses. These addresses are used especially by applications that support both IPv6 and IPv4. These addresses are called as IPv4 Mapped IPv6 Addresses. The format of these address is as follows:

Table H-4 Title not available (IPv4 Mapped IPv6 Address)

80 bits	16 bits	32 bits
zeros	FFFF	IPv4 address

Example:

::ffff:192.168.0.1

Aggregatable Global Unicast Addresses

The global unicast addresses are globally unique IPv6 addresses. This address format is very well defined in the RFC 2374 (An IPv6 Aggregatable Global Unicast Address Format). The format is:

Table H-5 Title not available (Aggregatable Global Unicast Addresses)

3	13	8	24	16	64 bits
FP	TLA ID	RES	NLA ID	SLA ID	Interface ID

whereFP = Format prefix. Value of this is “001” for Aggregatable Global unicast addresses.TLA ID = Top-level Aggregation Identifier.RES = Reserved for future use.NLA ID = Next-Level Aggregation Identifier.SLA ID = Site-Level Aggregation Identifier.

Interface ID = Interface Identifier.

Link-Local Addresses

Link-local addresses have the following format:

Table H-6 Title not available (Link-Local Addresses)

10 bits	54 bits	64 bits
1111111010	0	interface ID

Link-local address are supposed to be used for addressing nodes on a single link. Packets originating from or destined to a link-local address will not be forwarded by a router.

Site-Local Addresses

Site-local addresses have the following format:

Table H-7 Title not available (Site-Local Addresses)

10 bits	38 bits	16 bits	64 bits
1111111011	0	subnet ID	interface ID

Link-local address are supposed to be used within a site. Routers will not forward any packet with site-local source or destination address outside the site.

Multicast Addresses

A multicast address is an identifier for a group of nodes. Multicast addresses have the following format:

Table H-8 Title not available (Multicast Addresses)

8 bits	4 bits	4 bits	112 bits
11111111	flags	scop	group ID

“FF” at the beginning of the address identifies the address as a multicast address.

The “flgs” field is a set of 4 flags “000T”. The higher order 3 bits are reserved and must be zero. The last bit ‘T’ indicates whether it is permanently assigned or not. A value of zero indicates that it is permanently assigned otherwise it is a temporary assignment.

The “scop” field is a 4-bit field which is used to limit the scope of the multicast group. For example, a value of ‘1’ indicates that it is a node-local multicast group. A value of ‘2’ indicates that the scope is link-local. A value of “5” indicates that the scope is site-local.

The “group ID” field identifies the multicast group. Some frequently used multicast groups are the following:All Node Addresses = FF02:0:0:0:0:0:0:1 (link-local)

All Router Addresses = FF02:0:0:0:0:0:0:2 (link-local)All Router Addresses = FF05:0:0:0:0:0:0:2 (site-local)