IPv6 Network - Multicast Scoping

More often than not, it’s necessary to limit the propagation of multicast packets. For instance, it wouldn’t be good if all routers connected to the Internet were to receive all the hello packets that OSPF routers use to find their neighbors. These packets are for use on the local subnet only. And the speech by the CEO should probably only be multicast throughout the company, rather than Internet-wide.

The four scope bits in the address are preceded by four “flag” bits. RFC 3513 only defines the use of the last of these to indicate whether the 112 bits that make up the rest of the multicast address are a permanent, well-known value assigned by the Internet Assigned Numbers Authority (IANA), or some locally determined value. If the bit is set to zero, the 112-bit value is IANA-assigned. If the bit is set to one, the multicast address is “transient.” So ff12::/16 is the prefix for transient link-local multicast use, while ff0e::/16 is the prefix for a permanent global multicast addresses.

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