Classification of IPv6 Addresses
An IPv6 address is represented as 128 bits and is classified into several types depending on its purpose and scope. Unlike IPv4, there is no classful division (Class A–E); instead, the type of an address is determined by its leading bit pattern (prefix). Representative types include the global unicast address, unique local address (the private-address equivalent), loopback address, link-local address, and multicast address.
Allocation of the Address Space
| Type | Prefix | Purpose |
|---|---|---|
| Global Unicast Address | 2000::/3 | A unicast address routable on the internet. |
| Unique Local Address | fc00::/7 | An address for environments not directly connected to the internet, such as an organization’s internal network (the private-address equivalent). |
| Link-Local Address | fe80::/10 | An address valid only within the same link. |
| Multicast Address | ff00::/8 | An address used for one-to-many communication (the equivalent of Class D in IPv4). |
| Loopback Address | ::1/128 | An address that refers to the host itself. |
| Unspecified Address | ::/128 | A special address indicating that no address has been determined. |
Global Address (Global Unicast Address)
This is an address whose uniqueness is guaranteed across the internet and that can be routed directly on the internet. The 2000::/3 range is allocated by IANA through the Regional Internet Registries (RIRs) to ISPs and organizations, and it is typically composed of a 64-bit network prefix in the upper bits and a 64-bit interface ID in the lower bits.
Unique Local Address (Private-Address Equivalent)
This is an address defined for use within local networks that are not directly connected to the internet, such as inside an organization. It occupies a position equivalent to IPv4’s private addresses (RFC 1918) and is not routed on the internet.
| Item | Value |
|---|---|
| Prefix | fc00::/7 |
| Local (L) bit = 1 (in practical use) | fd00::/8 |
| Local (L) bit = 0 (reserved for future definition) | fc00::/8 |
| Global ID | A 40-bit random value that, in practice, avoids collisions between organizations. |
Loopback Address
This is an address reserved to refer to the host itself. Packets addressed to the loopback address are processed internally by the host without passing through a network interface. Unlike IPv4’s 127.0.0.0/8, IPv6 assigns only a single address per host.
| Item | Value |
|---|---|
| Address | ::1/128 |
Link-Local Address
This is an address valid only within the same link (the same segment). It is automatically assigned to every IPv6-enabled interface and is used for control communication within the link, such as Neighbor Discovery and router discovery. Packets addressed to or originating from a link-local address are not forwarded beyond a router.
| Item | Value |
|---|---|
| Prefix | fe80::/10 |
| Range in practical use | fe80::/64 (the lower 64 bits are used as the interface ID) |
Multicast Address
This is an address used for one-to-many communication, corresponding to Class D in IPv4. It is also used as a replacement mechanism for IPv4 broadcast, and is characterized by having flag and scope fields embedded within it.
| Item | Value |
|---|---|
| Prefix | ff00::/8 |
| Example: all-nodes multicast (link-local) | ff02::1 |
| Example: all-routers multicast (link-local) | ff02::2 |
Unspecified Address
This is a special address indicating that an address has not yet been determined. It is used during communication that temporarily lacks a source address, such as in the process of Duplicate Address Detection (DAD), and it cannot be used as a packet’s destination.
| Item | Value |
|---|---|
| Address | ::/128 |
Related RFCs
| RFC | Title | Summary |
|---|---|---|
| RFC 4291 | IP Version 6 Addressing Architecture | The original specification for IPv6 addressing. Defines the global unicast, link-local, loopback, unspecified, and multicast address types. |
| RFC 4193 | Unique Local IPv6 Unicast Addresses | Defines fc00::/7 as the unique local address range (the private-address equivalent). |
| RFC 4007 | IPv6 Scoped Address Architecture | Defines the zone concept for scoped addresses such as link-local addresses. |
| RFC 3587 | IPv6 Global Unicast Address Format | Defines the format of the global unicast address (network prefix and interface ID). |
| RFC 6890 | Special-Purpose IP Address Registries | Specifies the registry for special-purpose addresses in both IPv4 and IPv6. |