This is a bitmap of up to 32 sets of 32 bits each (128 octets), which defines which source ports are allowed to forward packets to which destination ports. Within each set of four octets, each bit is set if a packet with a matching destination address may be forwarded to that port.Since only the ES/1's ports 2 through 21 are available for bridging, all unused bits as well as the first 8 octets as well as the last 40 octets are meaningless.