B. Theory of network monitoring.
IEEE802 agreement is commonly used in the Internet.
This agreement defines the host adopts the subnet
broadcast the way with the data transmission, any host
sends packets which will be broadcast through the subnet,
that is, a single host data can be received by other hosts
within the same subnet. In normal Settings, the network
card of the host filter each arrival packets, only put the
packets whose destination address is the native into the
receive buffer, and discard other packets. But if change
your card receiving mode setting to promiscuous mode to
change the filtering strategy, network card can receive
each packet going through to realize the function of