Giddings 502-03814-60
層
如果把這個集線器弄得更智能一些，只發給目標MAC地址指向的那臺電腦，就好了。
雖然只比集線器多了這一點點區別，但看起來似乎有智能了，你把這東西叫做交換機。也正因為這一點點智能，你把它放在了另一個層級，數據鏈路層。
如上圖所示，你是這樣設計的。
交換機內部維護一張MAC地址表，記錄著每一個MAC地址的設備，連接在其哪一個端口上。
|
MAC地址
|
端口
|
|
bb-bb-bb-bb-bb-bb
|
1
|
|
cc-cc-cc-cc-cc-cc
|
3
|
|
aa-aa-aa-aa-aa-aa
|
4
|
|
dd-dd-dd-dd-dd-dd
|
5
|
假如你仍然要發給B一個數據包，構造了如下的數據結構從網口出去。
到達交換機時，交換機內部通過自己維護的MAC地址表，發現目標機器B的MAC地址bb-bb-bb-bb-bb-bb映射到了端口1上，于是把數據從1號端口發給了B，完事~
你給這個通過這樣傳輸方式而組成的小范圍的網絡，叫做以太網。
當然最開始的時候，MAC地址表是空的，是怎么逐步建立起來的呢？
假如在MAC地址表為空是，你給B發送了如下數據
由于這個包從端口4進入的交換機，所以此時交換機就可以在MAC地址表記錄條數據：
MAC：aa-aa-aa-aa-aa-aa-aa
端口：4
交換機看目標MAC地址（bb-bb-bb-bb-bb-bb）在地址表中并沒有映射關系，于是將此包發給了所有端口，也即發給了所有機器。
之后，只有機器B收到了確實是發給自己的包，于是做出了響應，響應數據從端口1進入交換機，于是交換機此時在地址表中更新了條數據
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