IPv6 Multicast

IPv4, the previous and widely used version of the internet protocol, was able to broadcast data to multiple devices. However, with IPv6, the latest internet protocol, broadcast has been replaced with multicast.

What is broadcasting?

For a computer network, broadcasting is sending data or a message not needing a response to all network users. A computer in a local network can send data to all other computers in the network at the same time. The main use of broadcasting is for the distribution of information without the need to send it multiple times.

A special type of IP address is needed for broadcasting and replaces the addresses of any recipients. A broadcast is structured to connect the devices of a network.

An example of calculating the broadcast address for an IPv4 host using an address of 184.12.0.0/12:

184.12.0.0/12 bitwise ORed with 0.15.255.255 = 184.271.255.255

Why replace with multicast?

Multicast is similar to broadcast in that one device can send data to multiple devices simultaneously. However, multicast isn’t constrained by a local, or even global, network. An IPv6 multicast address defines this network, which is known as a multicast group. As long as a device is signaling to join the multicast group, it can receive data multicasted to said group.

IPv6 multicast address have the prefix ff00::/8. This is equivalent to the IPv4 broadcast address 224.0.0.0/4. Since multicast addresses can’t ever be a source address, any data sent must have a unicast source address.

Examples of Multicast

Within multicast addresses are both well-known and solicited-node multicast addresses.

Well-known multicast addresses are multicast addresses reserved and assigned to specific device groups. These addresses are denoted with the prefix ff00::/12. Examples of well-known multicast addresses are:

• ff02::1 – All IPv6 devices
• ff02::2 – All IPv6 routers
• ff02::5 – All OSPFv3 routers
• ff02::a – All EIGRP (IPv6) routers

Solicited-node multicast addresses are similar to broadcast addresses for IPv4. Automatically created by a special mapping of a device’s unicast address, the address for solicited-node multicast is ff02:0:0:0:0:1:ff00::/104.