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Independent mode ： Computers are independent of each other .
With the development of the times , There is an increasing need for computers to communicate with each other , Sharing software and data , That is, multiple computers work together to complete the business , There is network interconnection .
Network interconnection ： Connect multiple computers together , Complete data sharing .
The essence of data sharing is Network data transmission , That is, computers transmit data through the network , Also known as Network communication .
Depending on the size of the network interconnection , Can be divided into LAN and WAN .

One 、 LAN and WAN

1.1 LAN LAN

LAN , namely Local Area Network, abbreviation LAN.
Local That is, it identifies that the LAN is local , A locally formed private network
The hosts in the LAN can easily communicate with each other , Also known as Intranet ; There is no connection between LAN and LAN , It's impossible to communicate .

There are many ways to build a network through a LAN ： Through the router / These switches are called local area networks .

• Direct connection based on network cable
  
  Connect the three main engines with two wires , These three hosts constitute a LAN .

• Based on switch
  
  The network ports on the switch are peer-to-peer .( They all have the same mouth ) The effect is to set up a LAN with the devices plugged into it . This Hosts inside the LAN can access each other , Switch is to build several devices into a LAN .

• Router based construction
  
  There are actually two types of ports in the router ：WAN Mouth and LAN mouth .
  Which is inserted in LAN Equipment on the mouth , In a LAN .
  adopt WAN The port is connected to another lan ;
  The router is connected to two LANs (LAN The mouth is a ,WAN Another one ).

1.2 Wide area network

Wide area network , namely Wide Area Network, abbreviation WAN.
Through the router , Connecting multiple LANs , Physically form a wide range of networks , A wide area network is formed . All LANs within the WAN belong to their subnets .

Wan is actually between LAN and WAN , There are no clear boundaries . Think of a larger lan , It can be called " Wide area network "
The world's largest wan , be called Internet( The Internet ).

Two 、 Network communication foundation

The purpose of network interconnection is Network communication , That is, network data transmission , A little bit more specific , It is between different processes in the network host , Data transmission based on network .
that , In the formed network , How to determine from which host , Transfer data to that host ？ That's what you need to use IP Address to identify .

2.1 IP Address

IР Address : Describes the location of a host on the network （ Locate a network location of the host ）.( It's equivalent to the receiving address when buying express )

Format ：
IP The address is a 32 The binary number of bits , This is not convenient for people to read and remember , It is usually divided into 4 individual “8 Bit binary number ”（ That is to say 4 Bytes ）, Such as ：01100100.00000100.00000101.00000110.
Usually use “ dotted decimal ” In the way of , namely a.b.c.d In the form of （a,b,c,d All are 0~255 A decimal integer between ）. Such as ：100.4.5.6.

special IP：
127.* Of IP The address is used for local loopback (loop back) test , Usually 127.0.0.1（ Loopback IP , Represent your own host ）.

2.2 Port number

Port number : Describes an application on a host .( Equivalent to the recipient's phone number )

Format ：
The port number is essentially a 2 Bytes (16 position ) Of unsigned integers （0~65535 Number of ranges ）, In network communication , The process can bind a port number , To send and receive network data .

Two different processes , Cannot bind the same port number , But a process can bind multiple port numbers .

3306 mysql Default port number .
When the server program starts , You need to bind a port number , So that the client program can access .

Well known port number : hold 0-1024 These port numbers , It has been divided into some specific functions .
Many network services are very commonly used , A very wide range of services , For better management, assign some special port numbers to these services , It's not mandatory , It's just advice .

such as ：
80： http The server
443 ：https The server
22： ssh
23： ftp
If you deploy yourself http The server , You can bind him to 80, It can also be bound to other ports . image java Well known in http The server tomcat, The default port used is not 80 It is 8080.

2.3 agreement

agreement , Short for network protocol , Network protocol is network communication （ Network data transmission ） A set of conventions that all network devices passing through must follow together 、 The rules . How to establish a connection 、 How to identify each other, etc . Only by following this Agreement , Computers can communicate with each other .
agreement （protocol） Finally, it is reflected in the format of data packets transmitted on the network .

Why an agreement is needed ？

When it comes to network communication , In essence, it transmits optical and electrical signals ; Through the frequency of the optical signal ( High frequency / Low frequency ) The level of the electrical signal ( High level / Low level ) To express 0 and 1. To deliver all kinds of different messages , We need to agree on the data format of both parties .

2.4 Protocol layer

The process of network communication is actually very complicated , There are a lot of details . If only one agreement is passed , To agree on all the details , This agreement will be huge , complex ; A better way is to put a large and complex agreement , Split into several small , A simpler agreement , Each agreement , Responsible for part of the work .

Protocol layering is similar to when making a phone call , Define different levels of protocols ：

Our agreement has only two layers ; But the actual network communication will be more complex , It needs to be divided into more levels .

The benefits of layering ：

benefits 1: Each layer protocol does not need to understand the details of other layer protocols .( Better packaging )

Caller , You don't need to understand how the phone works , Can complete the operation of making a phone, the person who makes a phone , You don't need to be a language master .

benefits 2: Replace the protocol of the corresponding layer with another protocol .( Better decoupling )

Caller , You don't have to use a wired phone , You can use a wireless phone .
Caller , You can also use English , Do not use Chinese 》

OSI Seven layer network model

OSI： namely Open System Interconnection, Open Systems Interconnection .

• OSI The seven layer network model is a logical definition and specification ： Logically divide the network into 7 layer .
• OSI The seven layer model is a framework design method , Its main function is to help different types of hosts to achieve data transmission ;
• Its greatest advantage is Service 、 The three concepts of interface and protocol are clearly distinguished , The concept is clear , The theory is also relatively complete . Through seven hierarchical structure models, the reliable communication between different systems and different networks can be realized .
  
  OSI The seven layer model is both complex and impractical ： therefore OSI The seven story model didn't land 、 Realization .
  When actually building a network , Just to OSI Partial layering in seven layer model design , That is, the following TCP/IP Five floors （ Or four floors ） Model to achieve .

TCP/IP Five floors （ Or four floors ） Model

TCP/IP Is a synonym for a group of agreements , It also includes many agreements , Make up the TCP/IP Protocol cluster .
TCP/IP The communication protocol uses 5 The hierarchy of layers , Each layer calls the network provided by its next layer to fulfill its own needs .

The following four layers of code logic are implemented by the operating system, driver and hardware .
From a global perspective , Think it's 5 layer , But the bottom physical layer describes the hardware device ,( It has nothing to do with software , Far away from the procedural ape ), At this time, it is considered to be the fourth floor .

The physical layer : Hardware equipment in network communication （ Ethernet cable / network card …）, Conventions for hardware devices , This is the scope of the physical layer protocol , It is necessary to ensure the communication between all hosts and network devices , They all match each other .

The physical layer is responsible for light / How electrical signals are transmitted . For example, the current Ethernet network cable ( Twisted pair Line )、 The coaxial cable used in early Ethernet ( Now it's mainly used in cable TV )、 Optical fiber , current wifi The use of electromagnetic waves in wireless networks belongs to the concept of physical layer . The capacity of the physical layer determines the maximum transmission rate 、 transmission distance 、 Anti interference, etc . A hub （Hub） Working on the physical level .

Data link layer : Be responsible for the completion of adjacent （ A network cable is connected ） Communication between two devices .[ Local ]

Responsible for the transmission and identification of data frames between devices . For example, drivers of network card devices 、 Frame synchronization ( That is to say, what signal is detected from the network cable is counted as the beginning of a new frame )、 Collision detection ( Automatically resend if a conflict is detected )、 Data error checking, etc . There's Ethernet 、 Token ring network , wireless LAN Equal standard . Switch (Switch) Working at the data link layer

The network layer : be responsible for Point to point communication .[ overall situation ]
Any node in the network , Communication to any node ( Not necessarily adjacent , More refers to non adjacent ), The network layer is responsible for between these two points , Plan a suitable route .( The actual network environment structure is very complex . There is more than one route between two points . We need the most appropriate one from the planning office ).

Transport layer : be responsible for End to end communication .
Start and end , Just focus on the results ( Whether the data arrives or not ), Don't focus on the process ( Don't care which way the data goes )

For example, when buying express, you need to fill in your recipient's address and recipient's name . The merchant should send the express to me according to this address , Me and the merchant , All focus on the results , Don't focus on the process , Express companies should pay attention to the process in the middle .

application layer ： Closely related to applications .( The data you transmitted , What is it for , Different applications have different uses )

For example, one day I bought a bed brush online ：
merchants , Standing on the transport layer ： Consider whether this thing can be sent to me .
Courier Services Company ： Plan the route at the network layer .
Express little elder brother ： Stand on the data link layer , Ride an electric car to pull the goods to the distribution center .
Electric cars / Container trucks / The highway station is on the physical layer ： Provide the basis for transmission .
The above four steps are only considering how the package is transported , Regardless of what's in this package , I don't care what the things in the package are for . And I'm ： Stand on the application floor as the person who buys a bed brush , Is to hold a certain purpose / Purpose to buy .

The layer where the network device is located

For one station host , Its operating system kernel implements content from transport layer to physical layer , That is to say TCP/IP The lower four layers of the five layer model ;
For one station Router , It has achieved From the network layer to the physical layer , That is to say TCP/IP The lower three layers of the five layer model ;
For one station Switch , It has achieved From data link layer to physical layer , That is to say TCP/IP The lower two layers of the five layer model ;

Notice that we're talking about Switches and routers in the traditional sense , Also known as layer 2 switch （ Working in TCP/IP The lower two layers of the five layer model ）、 Three layer router （ Working in TCP/IP The lower three layers of the five layer model ）.

3、 ... and 、 Packaging and distribution

• Different protocol layers have different names for packets , stay The transport layer is called segment (segment), In the network layer, it's called datagram (datagram), stay The link layer is called frame (frame).
• When the application layer data is sent to the network through the protocol stack , A data header should be added to each layer of the protocol (header), be called encapsulation (Encapsulation).
• Header information It contains something similar to how long the first part is , load (payload) How long is , What is the upper layer protocol .
• Data is packaged into frames and sent to the transmission medium , After reaching the destination host, the corresponding header of each layer protocol is stripped , According to the first part of “ Upper layer protocol field ” The data is handed over to the corresponding upper layer protocol for processing .

3.1 encapsulation

for example , Use QQ Send a message to a classmate , user A On the keyboard , Entered a "hello", Press the send key .

application layer （QQ Applications ）

According to the user's input ,QQ According to the application layer protocol designed by the program ape , Construct the data into an application layer protocol message （ agreement ： It's a convention , message ： A set of data that complies with this Agreement ）. notes ：QQ Application layer protocol used , It's development QQ The programmer set it himself , Suppose that the current application layer protocol format is ：

The application layer protocol calls the... Provided by the operating system API( be called socket API), Put the data of the application layer , To the transport layer ( Has entered the operating system kernel ).

Transport layer （ Operating system kernel ）

According to the data just transmitted , Based on the currently used transport layer protocol , To construct a transport layer protocol message . The most typical protocol of transport layer is UDP,TCP. With TCP For example ：

TCP Datagram =TCP Headlines ＋ Data load (Payload, That is, a complete application layer data ).

TCP There is a lot of information in the header of , The most important , Namely **“ Source port " and “ Destination port ”**, Similar to sender's phone and recipient's phone .

You can simply put this structure TCP The process of message is regarded as a string splicing ( What we spell here is binary data ). Next, the datagram of this transport layer , To the network layer .

The network layer （ Operating system kernel ）

Got the complete transport layer datagram , According to the current network layer protocol ( for example IP), Package again , hold TCP Datagram structure causes IР The datagram , Here is the last protocol header ：

IP The datagram =IР Protocol header ＋ load ( complete TCP/UDP Datagram )

There is also a lot of important information in this header , One of the most important is Source IP And purpose IP, Similar to the sender's address , And the recipient's address . Then , Current network layer protocols , It will IP The datagram , To the data link layer .

Data link layer （ The driver ）

In just now IP Based on datagram , According to the protocol of the data link layer currently used , Give the datagram constructed as a data link layer . Typical data link layer protocols , be called “ Ethernet ”, It will be constructed into a " Ethernet data frame "：

Ethernet data frame = Frame head ＋IP The datagram ＋ Frame tail

The most important information is stored in the frame header ： What is the address of the device to be sent next ;
The end of the frame is responsible for checking the sum .

IP The address written in the agreement , Is the beginning and end .( Xi'an to angung town )
Ethernet data frame , The address written in the frame header , Is the address of the next adjacent node .( Xi'an and Changchun )
As the data is forwarded to the next device , The address in the frame header , It's always changing .
People in Xi'an , the address for this place , It says Xi'an / changchun ;
People in Changchun , the address for this place , It's about Changchun / Baicheng ;
People in Baicheng , the address for this place , It says Baicheng / An Guang town ;

Data link layer , The above data will be handed over to a physical layer .

The physical layer （ Hardware device ）

The work done by the physical layer , According to the Ethernet data frame just now ( In fact, it's a group 0 1) Take this 0 1 Become high and low level , Through the network cable . Or take the 0 1 Become high frequency / Low frequency electromagnetic waves , Through optical fiber / Spread out wirelessly .

Here we are , At this point, the data has left the current host , To the next device .
Next device , It could be a router / Switch / Other equipment .

3.2 Divide up

Distribution is the reverse process of packaging ：
encapsulation It's from the top down , The data is added to the protocol header in turn ( Package express );
Divide up It's from the bottom up ,, The data is removed from the protocol header at one time ( Demolition Express );

The physical layer ( Hardware device , network card )

host B Your network card senses a set of high and low levels , Will translate these levels into 0 1 A string of data , Then this string 0 1 Is a complete Ethernet data frame . The physical layer gives this data to the data link layer .

Data link layer ( drive )

The data link layer is responsible for parsing this data , Remove the head and tail of the frame , Take out the inside IP The datagram , Then give it to the network layer protocol .

The network layer （ Operating system kernel ）

Network layer protocol (IP agreement ) This data will be analyzed again , Get rid of IP Protocol header , Take out the inside TCP The datagram is then handed over to the transport layer ;

Transport layer （ Operating system kernel ）

Transport layer protocol (TCP agreement ) This data will be analyzed again , Get rid of TCP Headlines , Take out the inside TCP The datagram , Give it to the application layer .

application layer （ Applications QQ）

The application layer will call socket API Read the application layer datagram from the kernel , Then it is parsed according to the application layer protocol , Display in the window according to the analysis results .

The above discussion is only about the starting point and the ending point .A and B There are many routers and switches in the middle ？

No matter how complex the network is , The overall transmission process here is similar . It's just Keep repeating the process of packaging and distribution .