IPv6 MULTICAST ADDRESS VS IPv4 BROADCAST
Multicast enables you to send a single packet but still communicate with multiple systems on the network. Although IPv4 can employ multicast, in IPv6 multicast takes a much more prominent role. IPv6 does not have a broadcast address, which is not surprising considering the default subnet size is 2^64. Multicast is used in various ways to communicate with specific device groups, such as All Hosts on the local network or All Routers on the local network. In IPv6 all nodes are required to support multicast. Without multicast, many services that you need will simply not work. There’s a constant hum from the network on the multicast channels.
When IPv4 was specified about 30 years ago, there was no multicast standard for IP networks initially. A lot of network services, like DHCP and ARP, had to depend on broadcast to work. IPv6 is a new protocol and had multicast services from start, so network discovery functions now builds upon multicast instead. This means that hosts that run these servers listen to the multicast addresses and other hosts will not be bothered when a client sends IP packets to these addresses. Multicast also enables site-wide and global services where clients subscribe to a data stream.
ICMP messages to these multicast groups enable IPv6’s Neighbor Discovery to operate efficiently. Other groups include all DHCP Servers and all Mobile Agents. The increased reliance on multicast goes hand-in-hand with the increased dependence on ICMP. The functionality of the IPv6 Neighbor Discovery protocol corresponds to a combination of the IPv4 protocols: Address Resolution Protocol (ARP), Internet Control Message Protocol (ICMP) Router Discovery, and ICMP Redirect. IPv4 does not have a generally agreed on protocol or mechanism for neighbor unreachability detection. However, host requirements do specify some