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In the IT security interview application process, the network basic knowledge must meet the following points:

1. Understanding models: in particular, it is necessary to understand OSI, TCP/IP, Cisco and defense-in-depth network models.

2. Understand the issues related to the second layer: including topics related to collision domain, broadcast domain, switching and hub, port security and spanning tree.

3. Understand the issues related to layer 3: including IP routing protocols; internal routing protocols, such as RIPv1/v2 and OSPF; external routing protocols, such as BGP, subnetworks, supernetworks; Network address Translation (NAT) and IPv6

4. Understand the issues related to layer 4: including TCP and UDP, session establishment, headers and options, port address translation and common ports

5. Understand layer 5-7 related issues, including application vulnerabilities, Trojans / worms / viruses, content filtering and IDS/ISP/IDP.

Most of the content here will be explained in detail next.

What is the OSI model?

The scope of this question is so wide that it is difficult to answer in short language. The answer involves seven layers and related protocols of each layer. What is very important in the answer is the discussion of encapsulation concept. Physical layer: "bit stream", "data frame" of data link layer, "Datagram" of network layer, etc. Although these are very simple, they are very important for IT security job search. Look up or read books on the Internet by yourself.

What is the TCP/IP model and how does it relate to the OSI model?

Find it yourself that is very important in the foundation.

What is the standard framework of Cisco?

Core layer, distribution layer (aggregation layer), access layer, the concept of each layer needs to be understood

4. How to realize defense in depth in OSI model

This is a big problem for network engineers. Defense in depth can explain this concept from two different angles. First of all, from the perspective of hierarchical security mechanism, it is called defense in depth. Defense-in-depth is a standard methodology that emphasizes security requirements at three levels: management, technology, and business. Before layering, the establishment of defense in depth focuses more on policies and procedures than on the security mechanisms that actually protect resources. Defense in depth includes physical security and man-made security. As stated in the question, the OSI model is the basis for explaining the concept of defense in depth. Everyone should know that defense in depth means the use of multiple approaches and protection methods to achieve security. Defense in depth can be understood by discussing the protection taken by each layer of the OSI model. As a company, he hires a line inspector for physical lines (layer 1), sits port security on switches (layer 2), makes access lists on border routers (layer 3), establishes unregulated zones in firewalls and sets up packet filtering (layer 4) to set up IDS/IDP monitoring / blocking (layer 3-7), establishes SSL channels (layer 6), sets up content filtering (layer 7), and provides AAA services (layer 7). Conduct vulnerability scanning (fourth to seventh) and virus scanning (seventh layer), etc.

Fifth, why is the network based on layer?

All network models are layer-based. Why? In essence, the concept of ISO roll-out layer is to illustrate the logical function of the network. Each layer is independent and provides its own special services, so it has its own data structure (header / trailer). At each layer, the corresponding header and footer are processed, and the data content is pushed into the stack. In addition, considering different vendor environments, it is also necessary to provide a relatively independent platform.

What is the principle of spanning Tree Protocol, what is its purpose, and what are its types?

Spanning Tree Protocol is applied to layer 2. Its purpose is to eliminate network loops and achieve this goal by selectively opening and blocking ports to control network transmission. The following ellipsis

7. How many commonly used ports do you know

FTP port 21 Ssh service port 22

Tenet port 23

The way to view the port is:

C:\ Windows\ System32\ drivers\ etc\ servies can be viewed by opening it in notepad

How is the port security of the switch realized?

Port security is more and more widely used in today's enterprise private network, and the resulting problems are more and more. Port security is a characteristic of the switch, which means that the switch can store MAC addresses on a given port, and only traffic from that address is allowed by the switch. If a device with a different MAC address is trying to connect, the switch disables the port and prompts the network administrator. Although it has been proposed that port security limits mobile office, it provides an excellent security mechanism to control network access.

IX. The difference between routing protocols and routing protocols is

What is access control?

11. What is the minimum authority?

12. How to define confidentiality?

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