This object indicates which ports belong to the same multicast group, on a per-VLAN basis. Each multicast group has an octet string which is indicated by a port map. The most significant bit represents the lowest numbered port, and the least significant bit represents the highest numbered port. Thus, each port of the switch is represented by a single bit within the value of this object. If that bit has a value of '1' then that port is included in the set of ports; the port is not included if its bit has a value of '0'(Note that the setting of the bits corresponding to the port from which a frame is received is irrelevant). The 4 octets represent one unit port according to its logical port. If the unit is less than 32 ports, other ports are not affected and are replaced with zeroes.