Siemens G24000-A3-M804-A3 產品說明
：在A上要設置默認網關
上一步A通過是否與C在同一個子網內，判斷出自己應該把包發給路由器，那路由器的IP是多少呢？
其實說發給路由器不準確，應該說A會把包發給默認網關。
對A來說，A只能直接把包發給同處于一個子網下的某個IP上，所以發給路由器還是發給某個電腦，對A來說也不關心，只要這個設備有個IP地址就行。
所以默認網關，就是A在自己電腦里配置的一個IP地址，以便在發給不同子網的機器時，發給這個IP地址。
僅此而已！
路由器如何知道C在哪里？
：路由表
現在A要給C發數據包，已經可以成功發到路由器這里了，最后一個問題就是，路由器怎么知道，收到的這個數據包，該從自己的哪個端口出去，才能直接（或間接）地最終到達目的地C呢。
路由器收到的數據包有目的IP也就是C的IP地址，需要轉化成從自己的哪個端口出去，很容易想到，應該有個表，就像MAC地址表一樣。
這個表就叫路由表。
至于這個路由表是怎么出來的，有很多路由算法，本文不展開，因為我也不會哈哈~
不同于MAC地址表的是，路由表并不是一對一這種明確關系，我們下面看一個路由表的結構。
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目的地址
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子網掩碼
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下一跳
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端口
|
|
192.168.0.0
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255.255.255.0
|
|
0
|
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192.168.0.254
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255.255.255.255
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|
0
|
|
192.168.1.0
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255.255.255.0
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1
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192.168.1.254
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255.255.255.255
|
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1
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我們學習一種新的表示方法，由于子網掩碼其實就表示前多少位表示子網的網段，所以如192.168.0.0（255.255.255.0）也可以簡寫為192.168.0.0/24
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目的地址
|
下一跳
|
端口
|
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192.168.0.0/24
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|
0
|
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192.168.0.254/32
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|
0
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192.168.1.0/24
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1
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192.168.1.254/32
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1
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這就很好理解了，路由表就表示，192.168.0.xxx這個子網下的，都轉發到0號端口，192.168.1.xxx這個子網下的，都轉發到1號端口。下一跳列還沒有值，我們先不管
配合著結構圖來看（這里把子網掩碼和默認網關都補齊了）圖中&筆誤，結果應該是.0
剛才說的都是IP層，但發送數據包的數據鏈路層需要知道MAC地址，可是我只知道IP地址該怎么辦呢？
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