Vivid metaphor (Repost on conflicting domains, broadcast domains, hubs, and switches)

 A lan is like a building. Each person (like a host) has his/her own room (the room is like a nic, and the room number is the physical address, that isMacAddress), the people inside (the host) have one walkie talkie. Because they work on the same channel, one person can speak and others can hear it. This is broadcast (send an information package to all hosts ), only the target will respond. Even though others hear it, they will ignore it (discard the package), and all these walkie talkie devices that can hear the broadcast will become a broadcast domain. These walkie talkie are hubs (HubEvery walkie talkie is like a port on the hub. Everyone knows that the walkie talkie cannot listen when talking. You must release the intercom key to listen, this work mode can only receive or send at the same time is half duplex. In addition, only one person can speak on the walkie talkie at the same time. If two or more people talk together, there will be conflicts and no one can hear clearly. Therefore, this constitutes a domain of conflict.

Broadcast domain(Broadcast domain): A set of devices in the network. That is, all devices that the same broadcast package can reach become a broadcast domain. When one of these devices sends a broadcast, all other devices can receive the broadcast frame.HubAndSwitchAll ports are in a broadcast domain, and each port on the router is a broadcast domain.

One day, people in the building could not stand this low-efficiency communication, so they upgraded the device and changed it to an internal phone number (SwitchSwitch, Each phone number is equivalent to a port on the switch), and each person has an internal number (the logical address isIPAddress ). (Here I want to talk about it separately.IPAddress andMacAddress translation problems. Common L2 switches only recognizeMacAddress, which has a built-inMacAddress Table, and constantly maintain and update it to determine which port corresponds to the hostMacAnd the communication software we use is based onIP,IPAddress andMacAddress conversion is performedARPAddress Resolution Protocol .) At the very beginning, no one knows which number corresponds to, so to call someone, you must broadcast it first:"XXXWhat is your number?""My number isXXXX". In this way, you have the target number. All the internal numbers are added to the phone book in this way.MacAddress Table), you can dial the extension number directly next time without broadcasting. As we all know, the phone number is a point-to-point communication device and will not affect others. The conflict will only be restricted to the local device. The line of a phone number is equivalent to a conflicting domain. Only when the extension is connected in series, there will be a conflict between the extension and the host. This conflict will not affect other calls outside. The telephone number is like the port number on the vswitch. That is to say, each port on the vswitch has a conflict domain. Therefore, the entire large conflict domain is divided into several small conflict domains. In addition, the call can be answered at the same time, and the work mode is full duplex. This is one of the reasons why vswitches have better performance than hubs.