Collision domain
L/ B = 2 * d/v
d is distance,L is packet size,V is velocity,B is bandwidth
So 72B / 100Mbps = 2 * d/ (2 * 10^8 m/sec)
L is of 72B is minimum frame size to detect collision...
V is standard velocity for Ethernet...
So d is 576m (collision domain)
As per standard it ia taken 250m
For gigabit ethernet distance won't be decrease but 72B + padding is used...