CoAP also introduces two new communication patterns: multicast and Event Subscription or Observe as it is called in CoAP. Since CoAP is based on UDP and UDP supports multicasting, using the Internet Group Management Protocol, or IGMP, CoAP messages can also be multicast, at least when encryption is not used. This is done by sending CoAP messages to predefined multicast addresses.
For IPv4, the CoAP multicast address is 224.0.1.187. For IPv6, it's [FF02::FD] or [FF05:FD] for the link-local or site-local scope.
Multicasting not only allows for efficient distribution, it also allows clients to find CoAP devices with multicast support by multicasting requests. Multicasting a request and monitoring responses allows you to find new devices in the local network. Typically, multicasting (IGMP) is turned off in many gateways, however.
Care must be taken when using CoAP multicasting, since encryption is not allowed using the current versions of the standards. This is expected to change in the future.