Binary 11111111 at the start of the address identifies the address as being a multicast address. Multicast addresses have the following format:

Figure 1. Multicast address format

flgs is a set of 4 flags:

Figure 2. Flags in multicast address

• The 3 high-order flags are reserved, and must be initialized to 0.
• T = 0 indicates a permanently-assigned (well-known) multicast address, assigned by the Internet Assigned Number Authority (IANA).
• T = 1 indicates a non-permanently assigned (transient) multicast address.

Scope is a 4-bit multicast scope value used to limit the scope of the multicast group. Group ID identifies the multicast group, either permanent or transient, within the given scope.