最近在想问题“IP与MAC的作用”，以及很早之前学过的网络知识。发现了一个非常好的网站 Practical Networking (www.practicalnetworking.net/series)，所以记录一下这个网站以及我所学到的知识.
为什么要同时用到IP与MAC
先回顾一下OSI模型
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Layer 1 Physical - anything that carries 1’s and 0’s between two nodes.
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Layer 2 Data Link - group together 1’s and 0’s into chunks known as Frames.
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Layer 3 Network - responsible for packet delivery from end to end.
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Layer 4 – Transport - responsible for service to service delivery.
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Layer 5, 6, and 7 - Session, Presentation, and Application - handle the final steps before the data transferred through the network (facilitated by layers 1-4) is displayed to the end user.
在这其中， IP作用在Layer 3， MAC作用在Layer 2. 在数据的传输过程中，Source IP和Destination IP是一直不会变的。而Source MAC和Destination MAC地址会随着每一跳而改变。
换句话说
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Layer 2 uses MAC addresses and is responsible for packet delivery from hop to hop.
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Layer 3 uses IP addresses and is responsible for packet delivery from end to end.
其过程可以用这张图表示 (来源):
