IPv6 Address Types and Scopes
By Stephen Bucaro
IPv6 has three types of addresses, unicast, multicast, and anycast.
Unicast addresses are used for communicating between two nodes, a one-to-one transmission mode.
Multicast addresses are used for communicating between one and many nodes, a one-to-many transmission mode.
Anycast addresses are used for communuicating between one node and nearest node (in terms of routing distance) of multiple nodes.
• Multicast addresses start with FF, any other value at the start of the address (00 to FE) indicates
it is a unicast or anycast address.
IPv6 Unicast and Anycast addresses have the following scopes:
Global The scope is global (IPv6 Internet addresses).
Link-local The scope is nodes on the same subnet.
Site-local The scope is the organization (private site addressing).
Unicast Global Addresses
• The bits 001 in the leftmost address bits identifies the address as an IPv6 unicast global address.
IPv6 unicast global addresses are globally routable. The structure of an IPv6 unicast global address creates the
three-level topology as shown below.
The first 48 bits of a unicast global address is the global address. The first 24 bits of this global address, called
the TLA ID (Top level Aggregation Identifier) is administered by the IANA, who allocates them to Internet registries.
They are used for high level routing.
The last 24 bits of this global address, called the NLA ID (Next Level Aggregation Identifier) identifies a
specific customer site.
The next 16 bits of a unicast global address, called the SLA ID (Site Level Aggregation Identifier) is
assigned by the specific customer, and may be used to define as many as 65,536 subnets within an individual organization's site.
The last 64 bits of a unicast global address is assigned by the customer to a specific node on a specific subnet.
Unicast Site-local Addresses
IPv6 unicast site-local addresses are similar to IPv4 private addresses, the scope of a site-local
address being the internetwork of an organization's site.
• Unicast site-local addresses start with FEC0 (1111 1110 1100 0000).
The following illustration shows the structure of a site-local address:
The initial 48 bits are fixed (FEC0) bits are followed by a 16-bit Subnet ID field, which
provides up to 65,536 subnets in a flat subnet structure. Or the high-order bits of the Subnet ID
field can be subdivided to create a hierarchical routing infrastructure. The last field is a 64-bit
Interface ID field that identifies a specific node on a specific subnet.
Because site-local and global addresses have the same structure after the first 48 bits
(the 16-bit SLA ID of a global address and the 16-bit Subnet ID of a site-local address
both identifing a subnet of an organization's site) you can assign a specific subnet number
to identify a subnet that is used for both site-local unicast and global.