The basic working principle of Ethernet HUB is broadcast (broadcast) technology, that is, HUB from any port to receive an Ethernet package, it will broadcast this Ethernet packet to all other ports, HUB does not remember which MAC address hanging A port. The NIC (NIC) on the HUB port performs the corresponding action according to the function required by the packet, which is controlled by the layer 3, which does not handle the Ethernet HUB. The Ethernet HUB is only responsible for from one port The received Ethernet packet is broadcast to all other ports (the so-called broadcast means that the HUB sends the Ethernet packet to all other ports and does not mean that the HUB changes the packet to a broadcast packet).
The Ethernet packet contains the source MAC address (the Ethernet address in the NIC, 48 bits long, see Figure 1) and the destination MAC address. The MAC address is the same as the destination MAC address in the Ethernet packet. The action is defined by layer 3, which is independent of Ethernet HUB. Ethernet HUB only processes Ethernet packets, that is, only the MAC address. If the destination MAC address does not exist, or if the destination MAC address does not respond (ie, the upper layer software does not handle), Ethernet HUB does not know and does not handle it. It is like a postman, he is based on the envelope on the address of the letter, just pass, no matter what the letter and the recipient is a reply, or the recipient for some reason did not reply, which led to the sender anxious, has nothing to do with the postman. Only the postman can not find the address will be returned when the letter, Ethernet HUB regardless of the bounce, only responsible for forwarding.
HUB works: ① HUB from a port A will receive the packet sent to all ports
HUB works: ② non-broadcast packet, the address and the destination MAC address of the same station to respond to user A
HUB works: ③ broadcast package, all users are responding to user A