Welcome to read for reference, the author is limited, if it is helpful, unfortunately glorious
1.What's a protocol? protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt.--------------------- ---------------------------------------
2.Q: (How to connect to the Internet) How to connect end systems to edge router? * residential access nets (personal routing connection) * institutional access networks (school, company) (access to the Internet through collectively owned network systems such as schools, companies) * mobile access networks (mobile Access through the Internet) -------------------------------------------- --------------
3.Network basics-packets (Internet basics --- data packet concept) packets: When sending a large amount of data at one time, such as 10GB, the data will be sent before sending Divide into packets of varying sizes and send them in batches. -------------------------------------------------- -------
4. (Communication) packet switching technology: packet-switching: hosts break application-layer messages into packets. The host divides the application layer data into data packets for transmission. Here will test a calculation question, a simple division operation, and ask you the time it takes to transfer the data. The screenshot of the original text is as follows: 
5. Queuing and loss: First, after multiple hosts (which can be understood as computers and mobile phones) send data to the router, packets from different hosts need to form a queue like a queue, and then the router will Carry out data forwarding; second, the above sending method, when the queue space is full, but the host is still sending data to the router, data packet loss will occur.
6. Summary of the third section:
Explanation: (Refer to the blog: https://blog.csdn.net/flowingflying/article/details/50518631 )
(1) What is the Internet: In a rough way, I think Internet is due to many small networks (Subnet) A logical network formed by interconnection, and several computers (hosts) are connected to each subnet. The Internet aims to exchange information resources with each other. It is based on some common protocols and is formed through many routers and public Internet. It is a collection of information resources and resource sharing.
(2) ISP (Internet Service Provider);
(3) IXP: Internet Exchange Point. The ISP POP is connected to the Internet Exchange Point (IXP), and different ISPs converge on the IXP to access each other's network and exchange information. Networks interconnected by IXPs and private peering connections together form the backbone of the Internet.
(4) POP: point-of-present. Computers and corporate networks are connected to the ISP through a point of presence (POP), which is located at the edge of the ISP network and only provides services for specific areas. POP can provide local access points and authentication (password control) for multiple end users.
7. Section 4: Packet loss and time delay in Internet data transmission
(1) Page 41 explains the calculation formula and calculation method of delay time. This formula needs to be understood and memorized.
d nodal = d proc + d queue + d trans + d prop
(2) ThroughPut: Throughput, throughput refers to the number of successfully transmitted data per unit of time for networks, devices, ports, virtual circuits or other facilities (in terms of Measurement of bits, bytes, packets, etc.).
Section 5: IP protocol
Key reference pictures:
10. The hardware equipment used in each layer
<Reference URL: The difference between switch and router http://network.51cto.com/art/201505/477755.html,
router (router): belongs to the network layer, the main function is Determining the data receiving object in the Internet, that is, finding the recipient's address, is more complicated than the switch function;
switch: belongs to the link layer (DataLink), the main function is to complete the communication between adjacent computers. The switch uses the physical address or MAC address to determine the destination address for forwarding data. The router uses the ID numbers of different networks (that is, the IP address plus the port number) to determine the address for data forwarding. The IP address is implemented in software and describes the network where the device is located. Sometimes these third-layer addresses are also called protocol addresses or network addresses. The MAC address is usually built in the hardware, assigned by the network card manufacturer, and has been solidified in the network card, and generally cannot be changed. The IP address is usually automatically assigned by the network administrator or system.
RP repeater is a device that connects network lines, and is often used for bidirectional forwarding of physical signals between two network nodes. The repeater mainly completes the function of the physical layer, is responsible for transmitting information on the physical layer of two nodes, and completes the functions of signal replication, adjustment and amplification, so as to extend the length of the network. Due to the loss, the power of the signal transmitted on the line will gradually attenuate, and when the attenuation reaches a certain level, it will cause signal distortion, which will lead to reception errors. The repeater is designed to solve this problem. It completes the connection of the physical circuit, amplifies the attenuated signal, and keeps it the same as the original data. In general, both ends of the repeater are connected to the same media, but some repeaters can also complete the transfer of different media.
Summary of this section:
