最高的3比特标记为保留域,必须为0
T = 0表示为永久分配(“公认”)多播地址(由IANA分配)
T = 1表示为多播
T = 0 indicates a permanently-assigned ("well-known") multicast address, assigned by the global internet numbering authority.
T = 1 indicates a non-permanently-assigned ("transient") multicast address.
scope
4比特。用来标记此多播组的应用范围。可能的值有
0 reserved
1 node-local scope //只在节点范围内有效
2 link-local scope //只在链路范围有效
3 (unassigned)
4 (unassigned)
5 site-local scope //只在一个网络内有效
6 (unassigned)
7 (unassigned)
8 organization-local scope //只在组织内有效
9 (unassigned)
A (unassigned)
B (unassigned)
C (unassigned)
D (unassigned)
E global scope //全局有效
F reserved