* # Section if it could not touch, you can skip, look at the practical exercise portion of the back of the net, until there is a certain feeling, and then back to back studies of the OSI model.
On the topic of the network, it can be directly discussed in detail in a book, that book is called "computer network."
Called "computer network" a grasp of a lot of books, have a common problem is that they talk about theory, not that practical operation, and is also a common problem both brick home ~ ~ not a talking. (I recommend CISCO tutorial, because there are practical operation)
Like to explain the concept of a VLAN, it just gave him planted a Chinese translation of "virtual LAN", if you do not know what that sounds like a relationship with virtualization and SDN.
(In fact open a packet tracer, a drag pull switcher, the PC 3 sets, knock several commands to configure a VLAN10, VLAN20, 1,3 into port 2 into the port VLAN VLAN20, then PC1,2,3 1,2,3 access port, respectively.
PC1,3 configure the IP address 192.168.10.1/24,192.168.10.2/24,PC2 configure the IP address 192.168.20.1/24, then PC1,3 can ping each other through, PC2 and PC1,3 mutual ping fails, you will know that VLAN is used for network isolation. Two chapters devoted a text description, might as well do an experiment to directly. )
Therefore, we try to be popular way to explore the knowledge of the network, as well as to consolidate the concept through the actual operation.
OSI seven-layer model
No matter what this "computer network" teaching, research networks, are not open around the OSI model.
Personal understanding of the seven levels colloquially mainly to achieve the following:
Application Layer **: ** is the protocol used by applications, such as mail using POP3, SMTP, Telnet using the Telnet, to obtain the IP address of the DHCP, domain name resolution of DNS, web browsing http protocol; this part of the main agreement how is the prescribed application software to communicate.
** Presentation Layer: ** decided to show (encoded) in the form of data, as a movie can be sampled, quantized, coded as RMVB, AVI, a picture can be JPEG, BMP, PNG and so on.
Session layer **: ** a connection (session) for the ends of the communication entity, an authentication authorization and intermediate checkpoint record (unexpected interruption time for the session can be continued similarly HTTP).
**传输层：**将一个数据/文件斩件分成很多小段，标记顺序以被对端接收后可以按顺序重组数据，另外标记该应用程序使用的端口号及提供QOS。（不同的应用程序使用不同计算机的端口号，同样的应用程序需要使用一样的端口号才能正常通信）
**网络层：**路由选路，选择本次通信使用的协议（http、ftp等），指定路由策略及访问控制策略。（IP地址在这一层）
**数据链路层：**根据端口与MAC地址，做分组（VLAN）隔离、端口安全、访问控制。（MAC地址在这一层）处理VLAN内的数据帧转发，跨VLAN间的访问，需要上升到网络层。
**物理层：**将数据最终编码为用0、1标识的比特流，然后传输。（例如将题主头像的图片，变为一串01100111100这样的数字来表示）。
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类比两个人聊天的过程
如果上述的理论讲述还是有点太过学术，那我们尝试用两个人的聊天过程，来类比OSI七层模型。
**1、 应用层：**两个人有两张嘴（硬件），并且已经学懂了说话，可以用脑袋来组织好语言，来表达自己想表达的意思（相当于软件），并且能够通过脑袋产生的电流调动神经细胞，控制嘴巴肌肉张合，把话说出来。（非生物科班出身通信人想表达能够通过软件控制底层硬件工作的意思）。
**2、 表现层：**可以通过多种方式表现/表达同一个事情，例如：可以通过笑话、唱歌、念诗以及正常说话等多种表现形式表达，也可以是使用多种表达方式表达同一句话，例如关于iphone6的bigger than bigger的各种调侃。
**3、 会话层：**这层管认证与检查点，例如在街上碰到个很脸熟的老同学，你会先来一句：“你……是？韩寒吗？我是郭敬明啊，小学的时候全班最高那个啊”然后老同学会说：“哦，小郭？哗，真的十几年没有见过面了，呃……话说你这么多年来都没有长高过么？”然后经过“身份认证”，会话就建立起来了，否则你随便找个陌生人，通过不“认证”，就无法建立会话了。至于检查点，就像中间接了个电话，然后回来继续“嗯，刚才我们说到哪？”。
**4、 传输层：**在说一个很复杂的东西的时候，你无法很简单的把它说明白，所以你要将这个东西分段来说，例如先说二战的背景、过程、结果、总结，分开4个来说，通过将这个东西分段，能够提高效率、清晰地进行说明。
特别是当你不是按照顺序说，而是按照了例如背景（1）、结果（3）、过程（2）、总结（4）的顺序，听众需要用脑袋对其进行标记，并且按照标记重组1、2、3、4，才能更好地理解整个表达的意思。
**5、 网络层：**其实两个人当面聊天，这相当于点对点的通信了，两个人都在同一个子网内，根本不需要做路由选择的，用不到三层的通信。关键是协议的选择，我认为相当于语言，假设两个人都只有一种共同能够懂的语言，必须要使用这种语言两个人才能聊下去。这相当于要选用一样的协议进行封装来通信。否则就是牛头不对马嘴了。
**6、 数据链路层：**两个人面对面聊天，直接将内容讲给对方听即可，中间也没有一个交换机似的共享载体，除非改成三个人，其中一个是翻译，翻译需要将A的说话翻译完转述给B听，这就像交换机了。而另外关于ACL等安全与访问控制，粗略可以对应“什么可以讲，什么不可以讲，什么内容可以讲给哪个听众知道，不可以讲给哪个听众知道”的选择策略。
** 7, the physical layer: ** The last is the ultimate content encoded say. And express what words represent different layers, this correspondence such as "Hello", "you", and to use the second tone, the "good" character with a third tone; "single" word in the end is to read "dan" or for the surname of "shan"; "threshold" refers to the threshold of the door, still a matter of difficulty and other information coding. Finally, after the encoded output, by express words to express the meaning.
Why study the OSI model
In fact, the beginning of exposure to the OSI model, I was forced to look ignorant, knocking several commands can be solved things, why must study this arcane model.
As the face of further learning network, including packet capture, analyze unpacking, and then realized that due to the lack of theoretical knowledge, there is no clear a packet from Client A to Client B in the transmission process, in the end how experienced the process of. So even capture caught, but there was no way to start the analysis.
OSI model is like martial arts internal strength (nine positive magic, human resource configurations), Cisco Huawei simulator knock command is the external work (kind of stick method, Magic), although learning external work alone, it is possible in rivers and lakes the mix, but if there is a profound internal strength skills, to learn a large diversion of heaven and earth, tai chi and other martial arts of the time, you can get started quickly, more efficiently.