Hierarchical thinking
-decompose the complex process into several relatively single-function sub-processes
-the whole process is clearer, the complex problems are simplified
-it is easier to find the problem and solve the problem in a targeted manner. The physical layer of the
OSI seven-layer reference model 
(corresponding to the device network card , Bitstream bits)-establish
, maintain, and disconnect physical connections
Data link layer (corresponding to device switch/layer two switch, data frame frame)-establish
data connection (logical address/IP address), perform hardware address (physical address/MAC address) addressing, error checking and other functions.
-Layer 2 switch: data communication on the same network segment, IP address is not recognized
Network layer (corresponding to device router/layer three switches, data packet packet)-Perform
logical address addressing to achieve path selection between different networks
-Path selection criteria
1. The longest subnet mask length
2. The management distance in the routing protocol The shortest
3. The best path quality
4. The default route
The difference between a router and a switch
-the switch implements data communication on the same network segment
-the router implements data communication on different network segments
-the switch implements MAC address forwarding according to the MAC address table
-the router implements routing forwarding according to the routing table
Available ip address value range
-ipv4=2^32-2
-ipv6=2^128-2
Transport layer (corresponding to the equipment firewall, data segment fregment) -Define
the protocol port number for data transmission, as well as flow control and error checking
-transmission Layer protocol + port number corresponds to different services at the application layer
Transport layer protocol: 1. TCP, which requires safety and reliability, but time extension
2. UDP requires high rate and short time delay
Example: TCP+80=WEB service
TCP+25=SMTP service
TCP+110=POP3 service
According to which services the client needs, the ports sent to the specific destination port (server) are
divided into three categories, ranging from 1 to 65535
-the recognized port
and the destination port associated with the general network application are called recognized Port, range 1 to 1023
-registered port
Port 1024 to 49151 can be used as source port or destination port. Organizations can use these ports to register applications such as instant messaging
-private ports
Ports 41952 to 65535 are usually used as source ports
TP/IP protocol suite composition
Network layer: ICMP (network communication monitoring and information feedback)
-ping: source and target connectivity monitoring
-tracert: to detect the route and fault point of the source and target, generally used to locate and debug the fault
Igmp: Multicast
ARP: Address Resolution Protocol
-Known target IP, unknown target MAC
RARP:-Known
target MAC, unknown target IP The
four elements of data communication:
-source IP
-source MAC
-destination IP
-destination MAC
ARP is used when the target IP is known and the MAC address is unknown. For example: if PC1 wants to send a message to PC2, but only knows the IP of PC2 and PC2MAC is unknown, it will send a broadcast to the switch through ARP. When the switch receives the broadcast frame, it will broadcast unconditionally, and all hosts connected to the switch will be If they can receive this broadcast frame, they will compare whether their IP is consistent with the target. If they are inconsistent, they will discard it. If they are consistent, the reply message will be sent to the sender. They will naturally add their own MAC and IP in the process of returning the packet. When the sender receives data, it will check the receiver's MAC and IP address and put them into the ARP cache table.
Data encapsulation process 
Relationship between client and server
Domain Name Server (DNS)
-Provide the IP address or domain name of the website so that the host can connect to the
Telnet server
-Allow the administrator to log in to the host from a remote location, and then control the host's services in the same way as the local control
Email server
-use simple mail Transfer Protocol (SMTP), Post Office Protocol (POP3) or Internet Mail Access Protocol (IMAP)
-Used to send e
- mails from the client to the server over the Internet-Specify the recipient in the format of user@xyz.
Dynamic Host Configuration Protocol (DHCP) server
-Assign IP addresses, subnet masks, default gateways and other information service
Web servers to clients
-Hypertext Transfer Protocol (HTTP)
-Used to transfer information between Web servers
-Most web pages use HTTP to access
file transfer protocols (FTP) server
-a service for downloading and uploading files between the client and the server
Analogy between mobile communication and computer communication
The air interface is divided into three layers, the
first layer L1 physical layer and the
second layer L2 data link layer. The link link layer is further divided into a medium access control layer and a wireless link control layer.
The third layer L3 radio resource control layer
5G new wireless interface commonly known as NR (New Radio)