The ancient Western prisons would keep all prisoners in a large house. But managers quickly found that there were many disadvantages. As long as one person farted in the middle of the night, it would wake up all prisoners. Putting a bunch of people together can easily breed mass riots. In order to overcome the above drawbacks, the prison designed by the management later had many small rooms. A room holds up to ten prisoners.
Although the prisoners were kept in each cell, they could not lock them to exchange cigarettes and beer in private. If two prisoners are in the same cell, this exchange is very simple, and you can find the other party directly to trade. But if two prisoners are in different cells, how do they trade?
Router
Alice wants to use her cigarette in exchange for Bob's beer, but Bob is in the cell far away. As soon as Alice had the clever idea, he handed the cigarette to the Pharaoh (boss / jail), please forward it to Bob. Pharaoh lay on the iron fence, stretched out his right hand and handed the cigarette to Lao Zhang (boss / jail) in the next cell. Please forward it to Bob, Alice. Lao Zhang repeated the action of Pharaoh, passing the cigarette to the jail of the next cell. After N relays, the cigarette finally reached Lao Li (Boss / Head). Li knew that Bob was in his cell and threw the cigarettes at Bob. Later, Bob passed the beer back in a similar way. Eventually, the beer arrived in Alice's hands. At this time, the transaction communication was completed.
In the above, the characters Pharaoh, Zhang, and Li are special. They can join hands with the prison next door, but others do not have this privilege. Alice wants to go hand in hand with Lao Zhang next door and see Pharaoh can't kill her!
The prison head is actually the router in computer communication. Pharaoh, Lao Li, and Lao Zhang are routers, and their counterparts are Hosts, and Alice and Bob are hosts.
Layer 3 Switch
Some corrupt jailers are also slowly playing this kind of relay game, often more efficient than the relay in jail, because jailers can speed up.
Gateway
The roles that can usually complete the cargo relay are gateways. So whether it is a router or a Layer 3 switch, it can act as a gateway.
But the other way around is not appropriate. For example, the gateway is a router, which is obviously wrong. Because the gateway may also be a Layer 3 switch and a Layer 3 firewall. Because the Layer 3 firewall is also a gateway.
Some students have different opinions. In computer communications, not only gateways, but also bridges (layer 2 switches), hubs (physical layer), and signal amplifiers (physical layer) can complete the cargo relay. Isn't the conclusion very rigorous?
In the above story, just use an image story to let beginners have a perceptual understanding. In fact, the gateway is a Layer 3 relay device and can process IP packet headers because IP packet headers are located at Layer 3.
Bridge
The bridge (layer 2 switch), although it can also complete the signal cargo relay, only the Ethernet frame header will be processed. Because the Ethernet frame header is located at the layer 2, the layer 2 switch is a layer 2 relay device. If a switch can handle not only the Ethernet frame header but also the IP header, then it is already a Layer 3 switch and can also act as a gateway.
Hub / Signal Amplifier
The hub / signal amplifier, although it can also complete the signal cargo relay, only processes physical signals, but does not understand the content of the signals. The physical signal is on a layer, so the signal amplifier is a layer of relay equipment.
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