Scope zones

Each IPv6 address has a specific scope in which it is defined. A scope is a topological area within which the IPv6 address can be used as a unique identifier for an interface or a set of interfaces. The scope for an IPv6 address is encoded as part of the address itself. A unicast address can have a link-local or global scope. A multicast address supports:

Interface-local
Link-local
Subnet-local
Admin-local
Site-local
Organization-local
Global scopes

See Unicast IPv6 addresses and Multicast IPv6 addresses for more discussions about unicast and multicast scopes.

A scope zone is an instance of a given scope. For instance, a link and all directly attached interfaces comprise a single link-local scope zone. A scope zone has the following properties:

A scope zone consists of a contiguous set of interfaces and the links to which the interfaces are attached.
An interface can belong to only one scope zone of each possible scope.
A node can be connected to more than one scope zone of a given scope. For instance, a node can be connected to multiple link-local scope zones if it is attached to more than one LAN.
The scope zone for an IPv6 address is not encoded within the address itself, but is instead determined by the interface over which the packet is sent or received.
There is a single scope zone for IPv6 addresses of global scope which comprises all interfaces and links in the Internet.
Packets that contain a source or destination address of a given scope can be routed only within the same scope zone, and cannot be routed between different scope zone instances.
Addresses of a given scope can be reused in different scope zones.
Scope zones associated with the inbound and intended outbound interfaces are compared to determine whether packets containing a limited scope address (for example, an address of scope other than global) can be successfully routed.
Scope zone representations (zone indices) are valid only on the node where they are defined. The same zone can have separate representations in each node that belongs to that zone.

To identify a specific instance of a scope zone, a node assigns a unique scope zone index to each scope zone of the same scope to which it is attached.