in my company, we just update the network, replace hubs with switchs, and we just use one router in the network. one of the consultants suggests us to change the subnet mask on all computers from 255.255.255.0 to 255.255.248.0
why need to change the subnet mask? so strange.
yeah, i find the answer myself, so i post the answer and hope to help every one had read this post.
coz in the hub environment, when i (host A) send a message to B. then the message wil go to the hub first and the hub will broadcast my message to every host in the same subnet, so that the traffic is large, if at the same time C send a message and the message reach the hub as A's message reach, the collision occurs. so that we need to reduce the number of the host in each subnet in order to reduce the collision.
but after we change to switch environment, can be said, no collision will occur, so we allow large traffic in the subnet, so we can have more pc in one subnet.