# IPv6 - Mulcast Address

[TOC]

## 簡介

參考 [RFC4291](https://datatracker.ietf.org/doc/html/rfc4291#section-2.7)

An IPv6 multicast address is an identifier for a group of interfaces
   (typically on different nodes).  An interface may belong to any
   number of multicast groups.  Multicast addresses have the following
   format:

   |   8    |  4 |  4 |                  112 bits                   |
   +------ -+----+----+---------------------------------------------+
   |11111111|flgs|scop|                  group ID                   |
   +--------+----+----+---------------------------------------------+

      binary 11111111 at the start of the address identifies the address
      as being a multicast address.

有一些「預設」的功能

## `flgs` --- 決定功能

      flgs is a set of 4 flags:     |0|R|P|T|
                                    +-+-+-+-+

         The high-order flag is reserved, and must be initialized to 0.

         T = 0 indicates a permanently-assigned ("well-known") multicast
         address, assigned by the Internet Assigned Numbers Authority
         (IANA).

         T = 1 indicates a non-permanently-assigned ("transient" or
         "dynamically" assigned) multicast address.

而當中的 "well-known address" 可以在同一個 RFC 的 [`2.7.1. Pre-Defined Multicast Addresses`](https://datatracker.ietf.org/doc/html/rfc4291#section-2.7.1) 中找到。

## `scop` --- 群播的範圍

      scop is a 4-bit multicast scope value used to limit the scope of
      the multicast group.  The values are as follows:

         0  reserved
         1  Interface-Local scope
         2  Link-Local scope
         3  reserved
         4  Admin-Local scope
         5  Site-Local scope
         6  (unassigned)
         7  (unassigned)
         8  Organization-Local scope
         9  (unassigned)
         A  (unassigned)
         B  (unassigned)
         C  (unassigned)
         D  (unassigned)
         E  Global scope
         F  reserved