Like with Classical IP and ARP over ATM, a way must be defined to resolve IP multicast addresses to ATM equivalents. ATM has no direct equivalent to a group address, but a point-to-multipoint connection can be set up to all members. To get all the ATM addresses for the members of a host group, a MARS_REQUEST can be sent to the MARS with the Class D IP address. The MARS will respond with a list of all the hosts that are a member of the host group in the MARS Cluster. If the MARS has no mapping for the Class D address, it returns a MARS_NAK. The list of addresses is sent back to the MARS client using a string of MARS_MULTI messages, the last one containing a flag to indicate that it was the last one. Each MARS_MULTI message contains as much ATM addresses for the host group as possible within the limits of the MTU size of the underlying VCC.
The MARS changes its response to a request for address resolution (MARS_REQUEST), when a MCS is configured for an IP host group. If a MCS is configured, a cluster member will receive the list of ATM addresses of MCSs (usually one) when it sends a MARS_REQUEST. The MCS for that group, when it sends the same MARS_REQUEST, will get the list of ATM addresses of members.