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| 2.9 |  | IPv6 |
| 2.9.2 |  | IPv6 address format |

As defined first by RFC 1884 and later revised by RFC 2373, IPv6 addresses are 128-bit identifiers for interfaces and sets of interfaces, not nodes. Three general types of addresses exist:

- Unicast – An identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address.
- Anycast – An identifier for a set of interfaces that typically belong to different nodes. A packet sent to an anycast address is delivered to the nearest, or first, interface in the anycast group.
- Multicast – An identifier for a set of interfaces that typically belong to different nodes. A packet sent to a multicast address is delivered to all interfaces in the multicast group.

To write 128-bit addresses so that they are readable to human eyes, the IPv6 architects abandoned dotted decimal notation in favor of a hexadecimal format. Therefore, IPv6 is written as 32 hex digits, with colons separating the values of the eight 16-bit pieces of the address.

Under current plans, IPv6 nodes that connect to the Internet will use what is called an aggregatable global unicast address. This is the familiar counterpart to the IPv4 global addresses. Like CIDR enhanced IPv4, aggregatable global unicast addresses rely on hierarchy to keep Internet routing tables manageable. IPv6 global unicast addresses feature three levels of hierarchy:

- Public topology – The collection of providers that provide Internet connectivity.
- Site topology – The level local to an organization that does not provide connectivity to nodes outside itself.
- Interface identifier – The level specific to the individual interface of a node.

This three level hierarchy is reflected by the structure of the aggregatable global unicast address, which includes the following fields:

- Format Prefix (FP) field, 3 bits – The 3-bit FP is used to identify the type of address, unicast, multicast, and so on. The bits 001 identify aggregatable global unicasts.
- Top-Level Aggregation Identifier (TLA ID) field, 13 bits – The TLA ID field is used to identify the authority responsible for the address at the highest level of the routing hierarchy. Internet routers will necessarily maintain routes to all TLA IDs. With 13 bits set aside, this field can represent up to 8,192 TLAs.
- Reserved (Res) field, 8 bits – IPv6 architecture defined the Res field so that the TLA or NLA IDs could be expanded as future growth warrants. Currently, this field must be set to zero.
- Next-Level Aggregation Identifier (NLA ID) field, 24 bits – The NLA ID field is used to identify ISPs. The field itself can be organized hierarchically to reflect a hierarchy or multi-tiered relationship among providers.
- Site-Level Aggregation Identifier (SLA ID) field, 16 bits – The SLA ID is used by an individual organization to create its own local addressing hierarchy and to identify subnets.
- Interface ID field, 64 bits – The Interface ID field is used to identify individual interfaces on a link. This field is analogous to the host portion of an IPv4 address, but it is derived using the IEEE EUI-64 format. When this field is on LAN interfaces, the Interface ID adds a 16-bit field to the interface MAC address.

In addition to the global unicast address space, IPv6 offers internal network numbers, or site local use addresses. These are analogous to RFC 1918 addresses. If a node is not normally addressed with a global unicast address or an internal site local use address, it can be addressed using a link local use address, which is specific to a network segment.

| Web Links IP Version 6 Addressing Architecture http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc1884.html