计算机网络基础(翻译版)Computer Network Basic
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1、 Fundamentals of Communication Protocol Stack

1. Introduction to OSI protocol stack 2. Detailed explanation of TCP protocol 3. Detailed explanation of IP address 4. Gateway, DNS, ARP, and communication foundation
   2、 Exchange technology
2. Exchange basics 2. VLAN introduction 3. STP detailed explanation
   3、 Routing technology
3. Static routing 2. RIP protocol 3. OSPF protocol
   4、 Applied Technology
4. DHCP principle 2. Telnet principle 3. FTP protocol
   5、 Access Control Technology and NAT
5. ACL access control list 2. NAT network address translation
   1 Fundamentals of Communication Protocol
   Introduction to OSI Protocol Stack
   I. The OSI model is divided into seven layers:
   Physical layer – Data link layer – Network layer – Transport layer – Session layer – Presentation layer – Application layer
   II The OSI seven layer model enables reliable communication between different systems and networks through seven hierarchical structural models. Therefore, its main function is to help different types of hosts achieve data transmission. The node that completes the relay function is usually referred to as a relay system.
   Equipment on each layer:
   Physical layer: network card, network cable, hub, repeater, modem
   Data link layer: bridge, switch
   Network layer: router
   The gateway operates at or above the fourth layer transport layer
   VI. Functions of each device:
   Hub: A physical layer device that transmits information through broadcasting.
   Switch: used for packet exchange.
   Bridge: When a bridge works, it determines the forwarding port based on the MAC header, so it is obviously a device in the data link layer.
   Router: One function is to connect different networks, and the other function is to select the route for information transmission.
   2、 Exchange technology
6. Exchange Foundation
   The exchange technology in computer networks is actually how the hosts at both ends of the computer network transmit data packets through the network. For example, in the telephone system, two main switching technologies are used: circuit switching technology and packet switching technology.
   From the perspective of communication resource allocation, “switching” is to dynamically allocate the resources of transmission lines in a certain way.
7. VLAN Introduction
   VLAN is a virtual local area network, where a VLAN forms a logical subnet, i.e. a logical broadcast domain. VLANs work in the second and third layers of the OSI reference model, and communication between VLANs is completed through routing in the third layer.
   The role of VLAN: avoiding broadcast storms, ensuring security, and facilitating management
8. STP Explanation
   In order to improve network reliability, redundant links are usually used in switching networks. However. Redundant links can bring loop risks to switching networks, leading to broadcast storms and unstable MAC address tables, which in turn can affect the communication quality of users. The Spanning Tree Protocol (STP) can improve reliability while avoiding various issues caused by loops.
   The principle of STP protocol: Block a certain interface in the network (logically), achieve normal physical link connection, logically close (down), and avoid the harm of the loop.
   3、 Routing technology
9. Static routing
   A routing method where routing entries are manually configured rather than dynamically determined. Unlike dynamic routing, static routing is fixed and will not change, even if the network conditions have changed or been reconfigured. Generally speaking, static routing is added item by item to the routing table by the network administrator.
10. RIP protocol
    RIP is applied to the application layer of the OSI network’s seven layer model.. RIP is a distributed distance vector based routing protocol, which was introduced by Xerox in the 1980s and is mainly suitable for small-scale network environments.
    Working principle:
    Using neighbor’s routing table to establish its own routing table: When receiving a routing table from a neighbor, the router will update its routing table:
    Firstly, add 1 to the distance of the received route (i.e. the distance of one hop);
    Reuse the received routing table to modify one’s own routing table
11. OSPF protocol
    Open Shortest Path First (OSPF) was developed in 1989 to overcome the shortcomings of RIP. The principle of OSPF is simple, but its implementation is complex
    OSPF routing protocol is a link state routing protocol used for Internet Protocol (IP) networks. This protocol uses the Internal Gateway Protocol (IGP) of the Link State Routing algorithm and operates within a Single Autonomous System (AS)
    The OSPF protocol relies on five different types of packets to establish adjacency relationships and exchange routing information, namely greeting packets, database description packets, link state request packets, link state update packets, and link state confirmation packets
    4、 Applied Technology
12. DHCP principle
    DHCP Dynamic Host Configuration Protocol is a network protocol for a local area network that operates using the UDP protocol and has two main purposes: automatically assigning IP addresses to internal networks or network service providers, and serving as a central management tool for users or internal network administrators to manage all computers.
13. Telnet principle
    Relogin was developed for Berkeley UNIX and is a relatively simple and stable protocol that has been adopted by users outside the UNIX community. TELNET is a feature rich TCP/IP standard. It is the most widely used protocol in the TCP/IP protocol family. Provide a universal tool for accessing internet resources in a connected manner, allowing users to communicate with servers on a remote machine, supporting different physical terminals through a negotiation process, thereby providing great flexibility. The following diagram shows the typical structure of TELNET client and server.
    Working principle:
    TELNET maps terminals to a logical device using consistent standards, enabling data exchange between clients and servers across different types of terminals.
    The main body of the TELNET protocol consists of three parts:
    Ÿ The definition of Network Virtual Terminal (NVT);
    Ÿ Definition of operation negotiation;
    Ÿ Negotiate finite automata;
14. FTP protocol
    The File Transfer Protocol is a standard protocol used for file transfer over a network. It operates at the seventh layer of the OSI model and the fourth layer of the TCP model, which is the application layer. It uses TCP transmission instead of UDP. Before establishing a connection with the server, customers need to go through a “three handshake” process to ensure that the connection between the client and the server is reliable and connection oriented, providing reliable guarantees for data transmission.
    Working principle:
    It uses two TCP connections between two communicating hosts, one for data transmission; The other is the control connection, which is used to transmit control information (commands and responses). This idea of separating command and data transmission greatly improves the efficiency of FTP, while other client server applications generally only have one TCP connection.
    5、 Access Control Technology and NAT
15. ACL access control list
    Access Control List (ACL) is a packet filtering based access control technique that filters packets on an interface based on set conditions, allowing them to pass or drop. Access control lists are widely used in routers and layer three switches. With the help of access control lists, users can effectively control their access to the network, thereby maximizing network security.
    Access control lists have many functions, such as limiting network traffic and improving network performance; Control of communication flow.
16. NAT network address translation
    When some hosts within the private network have already been assigned a local IP address (i.e. a dedicated address used only within the private network), but want to communicate with hosts on the Internet (without encryption), the NAT method can be used. This method requires the installation of NAT software on the router connecting the private network (private IP) to the internet (public IP).
    NAT not only solves the problem of insufficient IP addresses, but also effectively avoids attacks from outside the network, hides and protects computers inside the network.
17. Broadband sharing: This is the biggest feature of NAT hosts.
18. Security protection: When a PC within NAT is connected to the Internet, the displayed IP is the public IP of the NAT host, so the client PC naturally has a certain degree of security. When external parties perform portscan, the source client PC cannot be detected.

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