Pv6 Address Scope Types

Similar to IPv4, a single source can address datagrams to either one or many destinations at the same time in IPv6.

NOTE RFC 4291, IPv6 Addressing Architecture, defines the IPv6 addressing architecture.

Following are the types of IPv6 addresses:

■ Unicast (one-to-one): Similar to an IPv4 unicast address, an IPv6 unicast address is for a single source to send data to a single destination. A packet sent to a unicast IPv6 address goes to the interface identified by that address. The IPv6 unicast address space encompasses the entire IPv6 address range, with the exception of the FF00::/8 range (addresses starting with binary 1111 1111), which is used for multicast addresses. The "IPv6 Unicast Addresses" section discusses the different types of IPv6 unicast addresses.

■ Anycast (one-to-nearest): An IPv6 anycast address is a new type of address that is assigned to a set of interfaces on different devices; an anycast address identifies multiple interfaces. A packet that is sent to an anycast address goes to the closest interface (as determined by the routing protocol being used) identified by the anycast address. Therefore, all nodes with the same anycast address should provide uniform service.

Anycast addresses are syntactically indistinguishable from global unicast addresses because anycast addresses are allocated from the global unicast address space. Nodes to which the anycast address is assigned must be explicitly configured to recognize the anycast address.

Anycast addresses must not be used as the source address of an IPv6 packet.

Examples of when anycast addresses could be used are load balancing, content delivery services, and service location. For example, an anycast address could be assigned to a set of replicated FTP servers. A user in China who wants to retrieve a file would be directed to the Chinese server, whereas a user in the Europe would be directed to the European server.

■ Multicast (one-to-many): Similar to IPv4 multicast, an IPv6 multicast address identifies a set of interfaces (in a given scope), typically on different devices. A packet sent to a multicast address is delivered to all interfaces identified by the multicast address (in a given scope). IPv6 multicast addresses have a 4-bit scope identifier (ID) to specify how far the multicast packet may travel.

KEY POINT

IPv6 has no concept of broadcast addresses; multicast addresses are used instead.

An IPv6 address is valid for a specific scope, which defines the types of applications the address is suitable for.

KEY POINT

A single interface may be assigned multiple IPv6 addresses of any type (unicast, anycast, and multicast).

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