Subnet mask tells the number of network bits and number of host bits in an IP. Number of ON bits = number of network bits Number of OFF bits= number of host bits   Wild Card mask is used to match the IP address So that any action can be performed on them like to drop those matched IPs. Number of OFF bits =  number of bits to be matched Number of ON bits = number of bits can be wild (any) ... (more)

THERE ARE PER PORT COLLISION DOMAIN IN SWITCHES.SO ON THE LEFT SIDE TWO SWITCHES HAVE TOTAL 4 PORTS. SO 4 COLLISION DOMAIN(on that part).

I WILL CORRECT IT . THERE ARE 9 COLLISION DOMAIN