This read-write OCTET STRING is a bitmap of up to 32 sets of 32 bits (128 octets total) defining which source ports are allowed to forward packets to which destination ports, with only ports 2 through 21 meaningful for bridging on this device. An administrator sets bits in it to implement port-based traffic isolation independent of the per-address classification in addrFlags. For example, the admin sets bits in addrPortMap to prevent port 5 from ever forwarding packets destined for port 10, isolating the two segments at the bridging layer.