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Basic Knowledge Tutorial to Explore Networking


A network consists of several nodes and links connecting these nodes, representing a set of objects and their relationships. Mathematically, a network is a graph, which is usually considered a weighted graph. In addition to mathematical definition, network has a specific physical meaning, that is, network is a model abstracted from a certain type of practical problem. In field of computing, a network is a virtual platform for transmitting, receiving and exchanging information, through which information from various points, surfaces and bodies is linked together to realize sharing of these resources. The network is most important invention in history of human development, which has improved development of technology and human society. Below are basic knowledge tutorials for understanding Internet, collected and organized by editor. Welcome to learn from them.

Understanding basic knowledge from network guide (1)

1) What is a link?

A link is a connection between two devices. It includes cable types and protocols used by one device to communicate with another.

2) What are layers of OSI reference model?

There are 7 OSI layers: physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer.

3) What is a backbone network?

A backbone network is a centralized infrastructure designed to distribute different routes and data across different networks. It also manages bandwidth and various channels.

4) What is a local network?

LAN is an abbreviation for Local Area Network. It refers to connection between a computer and other network devices located in a small physical location.

5) What is a node?

A node is a point where a connection occurs. It can be a computer or a device that is part of a network. Two or more nodes are required to form a network connection.

6) What is a router?

A router can connect two or more network segments. These are intelligent network devices that store information such as paths, hop counts, etc. in their routing tables. With this information, they can determine best route for data transfer. Routers operate at OSI network layer.

7) What is a two-dot link?

Indicates a direct connection between two computers on network. A peer-to-peer connection does not require any other network equipment other than connecting cables to network cards of both computers.

8) What is anonymous FTP?

Anonymous FTP is a way to give users access to files on public servers. Users who are allowed to access data on these servers do not need to identify themselves, onor must be logged in as anonymous guests.

9) What is a subnet mask?

The subnet mask is combined with IP address to identify two parts: extended network address and host address. Like an IP address, a subnet mask is 32 bits long.

10) What is maximum allowed UTP cable length?

One segment of UTP cable has an allowable length of 90 to 100 meters. This limitation can be overcome by using repeaters and switches.

11) What is data encapsulation?

Data encapsulation is process of breaking information into smaller, manageable chunks before it is sent over network. During this process, source and destination addresses are added to header, along with parity.

12) Describe network topology

Network topology refers to layout of a computer network. It shows physical location of devices and cables, and how they connect to each other.

13) What is


___ stands for Virtual Private Network, a technology that allows a secure channel to be created over a network such as Internet. For example, V___ allows you to establish a secure dial-up connection to a remote server.

14) Briefly describe NAT.

NAT stands for Network Address Translation. It is a protocol that allows multiple computers on a public network to share a single Internet connection.

15) How does network layer work in OSI reference model?

The network layer is responsible for data routing, packet switching, and network congestion control. Routers operate below this layer.

16) How does network topology influence your decisions when building a network?

The network topology determines which medium should be used to connect devices. It also serves as basis for materials, connectors and leads suitable for installation.

17) What is RIP?

RIP, short for Routing Information Protocol, is used by routers to send data from one network to another. It efficiently manages routing data by broadcasting its routing table to all other routers on network. It determines network distance in jumps.

18) What are ways to protect computer networks?

There are several ways to do this. Install a reliable and up-to-date antivirus program on all computers. Make sure firewall is installed and configured correctly. User authentication would also be very helpful. All this together forms a highly reliable network.

19) What is a network card?

Network Card is an abbreviation for Network Interface Card. This is connected to a PC to connect to Shenbei network. Each network card has its own MAC address, which identifies PC on network.

20) What is a WAN?

WAN stands for Wide Area Network. It is interconnection of geographically dispersed computers and devices. He counites networks located in different regions and countries.

21) What is importance of OSI physical layer?

The physical layer performs conversion of data bits into electrical signals and vice versa. This is a consideration and setup for network equipment and cable types.

22) How many layers are there in TCP/IP?

There are four layers: network layer, internet layer, transport layer, and application layer.

23) What are proxy servers and how do they protect computer networks?

A proxy server basically prevents external users from identifying IP address of internal network. Not knowing correct IP address or even determining physical location of network. A proxy server can make a network virtually invisible to outside users.

24) What is function of OSI session layer?

This layer provides protocols and methods for communicating between two devices on a network by holding sessions. This includes setting up session, managing exchange of information during session, and unwind process after session ends.

25) How important is implementation of a fault-tolerant system? Are there any restrictions?

Fault-tolerant systems ensure that data is always available. This is achieved by eliminating single points of failure. However, there are certain situations where this type of system will not be able to protect data, such as accidental deletion.

26) What does 10Base-T mean?

The number 10 means data transfer rate, in this case 10 Mbps. "Base" means main band. T stands for twisted pair, which is cable used for this network.

27) What is a private IP address?

Dedicated IP addresses are assigned for use on internal network. These addresses are used on internal network and cannot be routed on external public network. This ensures that there are no conflicts between internal networks, while a private IP address range can also be reused across multiple internal networks as they do not "see" each other.

28) What is NOS?

NOS or Network Operating System is specialized software whose main task is to provide a computer with a network connection so that it can communicate with other computers and connected devices.

29) What is DoS?

A denial of service or denial of service attack is an attempt to prevent users from accessing Internet or any other network service. This attack can come in many forms, consisting of a group of permanents. The usual way to do this is to reboot system server so that it can no longer handle legitimate traffic and is forced to reboot.

30) What is OSI and what role does it play in a computer network?

OSI (Open Systems Interconnection) As a reference model for data transmission. It has 7 levels andeach layer defines a specific aspect of how network devices connect and communicate with each other. One layer can deal with physical media being used, while another layer determines how data is actually transferred over network.

31) What is purpose of cable shielding and twisted pairs?

Its main purpose is to prevent crosstalk. Crosstalk is electromagnetic interference or noise that can affect data transmitted over a cable.

32) What are benefits of sharing addresses?

Address sharing provides inherent security benefits by using address translation rather than routing. This is because hosts on Internet can only see public IP address of external interface on computer providing address translation, and not private IP address on internal network.

33) What is a MAC address?

MAC, or Media Access Control, uniquely identifies devices on a network. It is also known as a physical address or Ethernet address. The MAC address consists of 6 bytes.

34) What is equivalent TCP/IP application layer or layers in terms of OSI reference model?

The TCP/IP application layer actually has three peers in OSI model: session layer, presentation layer, and application layer.

35) How to determine IP class for a given IP address?

By looking at first octet of any given IP address, you can determine if it is in class A, B, or C. If first octet starts with bit 0, address is in class A. If it starts with bit 10, address belongs to class B. If it starts with 110, then it is a class C network.

36) What is main purpose of OSPF?

OSPF, or Open Shortest Path First, is a link-state routing protocol that uses routing tables to determine best path for communication.

37) What is a firewall?

Firewalls are used to protect internal networks from external attacks. These external threats can be hackers who want to steal data, or computer viruses that can destroy data instantly. It also prevents other users from external networks from accessing private network.

38) Describe star topology

A star topology consists of a central hub connected to nodes. It is one of easiest to set up and maintain.

39) What is a gateway?

A gateway provides a connection between two or more network segments. This is usually computer that runs gateway software and provides translation services. This translation is key to allowing different systems to interoperate on network.

40) What are disadvantages of a star topology?

The main disadvantage of star topology is that after damage to centerhub or switch, entire network becomes unusable.

41) What is SLIP?

SLIP or Serial Line Interface Protocol is actually an old protocol developed in early days of UNIX. This is one of protocols used for remote access.

42) Give some examples of private network addresses., subnet mask, subnet mask, subnet mask

43) What is tracert?

Tracert is a Windows utility that can be used to trace data collection path from router to target network. It also shows number of hops completed during entire transmission route.

44) What is function of a network administrator?

A network administrator has many responsibilities that can summarize into 3 main functions: network setup, network settings, and network maintenance/troubleshooting.

45) Describe disadvantages of peer-to-peer networks.

When you access a resource that is shared by a workstation on a network, performance of that workstation is degraded.

46) What is a hybrid network?

A hybrid network is a network configuration that uses a client-server architecture and a peer-to-peer network.

47) What is DHCP?

DHCP is an acronym for Dynamic Host Configuration Protocol. Its main task is to automatically assign IP addresses to devices on network. It first checks next available address that is not already occupied by any device and assigns it to network device.

48) What is main purpose of ARP?

The main job of ARP or Address Resolution Protocol is to map known IP addresses to MAC layer addresses.

49) What is TCP/IP?

TCP/IP is an acronym for Transmission Control Protocol/Internet Protocol. It is a set of protocol layers designed to communicate across different types of computer networks (also called heterogeneous networks).

50) How to use a router to manage a network?

The router has a built-in console that allows you to configure various settings such as security and data logging. You can assign restrictions to your computer, such as resources you are allowed to access or specific times of day when you can surf Internet. You can even place restrictions on websites that are not visible on entire web.

51) If you want to transfer files between different platforms (e.g. UNIX system and Windows server), which protocol can be used?

Use FTP (File Transfer Protocol) to transfer files between these different servers. This is possible because FTP is platform independent.

52) What is use of a default gateway?

The default gateway provides means to connect a local network toexternal network. The default gateway used to connect to external network is usually port address of external router.

53) One way to secure your network is to use passwords. What is a good password?

A good password is not just letters, but a combination of letters and numbers. A password that combines uppercase and lowercase letters takes precedence over a password that uses all uppercase or all lowercase letters. Hackers should not easily guess passwords such as dates, names, favorites folders, etc.

54) What is correct connection speed for UTP cables?

Normal terminating impedance for unshielded twisted-pair network cables is 100 ohms.

55) What is netstat?

Netstat is a command line utility. It provides useful information about current TCP/IP connection settings.

56) How many network IDs are there in a Class C network?

For Class C networks, number of available network ID bits is 21. The number of possible network IDs is 2, increased to 21, or 2,097,152. The number of host IDs per network ID is 2, increased to 8 minus 2, or 254.


57) What happens if you use a cable longer than specified length?

Cables that are too long can cause signal loss. This means that transmission and reception of data will be affected due to reduction in signal length.

58) What are some common software problems that can cause network outages?

Problems related to software can be any or a combination of them:

- client-server problem

- Application Conflict

- configuration error

- Protocol Mismatch

- Security question

- Policy and User Rights Issues

59) What is ICMP?

ICMP is Internet Control Message Protocol. It provides messaging and communication for protocols in TCP/IP protocol stack. It is also a protocol for managing error messages used by network tools such as PING.

60) What is Ping?

Ping is a utility that allows you to test connectivity between network devices on your network. YouA device can be pinged using its IP address or device name (for example, computer name).

61) What is a peer-to-peer network (P2P)?

Peers are networks that do not respond to server. All PCs on this network are separate workstations.

62) What is DNS?

DNS stands for Domain Name System. The main function of this network service is to provide hostnames for resolving TCP/IP addresses.

63) What are advantages of optical fiber and other media?

The main advantage of optical fiber is that it is less susceptible to electrical interference. It also supports higher bandwidth, which means you can sendand get more data. There is also very little signal degradation over long distances.

64) What is difference between a hub and a switch?

The hub acts as a multiport repeater. However, as more and more devices connect to it, it will not be able to effectively manage traffic passing through it. Switches are best alternative that can improve performance, especially when high traffic is expected on all ports.

65) What network protocols does Windows RRAS support?

Supports three major network protocols: NetBEUI, TCP/IP, and IPX.

66) What are largest networks and nodes in class A, B, and C networks?

There are 126 possible networks and 16,777,214 hosts for class A

There are 16,384 possible networks and 65,534 hosts for class B

There are 2,097,152 possible networks and 254 hosts for class C

67) What is standard color sequence for straight cables?

Orange/White, Orange, Green/White, Blue, Blue/White, Green, Brown/White, Brown.

68) Which protocol belongs to application layer of TCP/IP protocol stack?

The following TCP/IP application layer protocols are FTP, TFTP, Telnet, and SMTP.

69) You need to connect two computers to exchange files. Can this be done without using a hub or router?

Yes, you can connect two computers with one cable. In this case, a crossover cable can be used. In this setup, transmit data pin of one cable is connected to receive data pin of other cable and vice versa.

70) What is ipconfig?

Ipconfig is a utility commonly used to determine address information for computers on a network. It can display physical addresses as well as IP addresses.

71) What is difference between straight and crossover cables?

Straight-through cables are used to connect your computer to a switch, hub, or router. Crossover cables are used to connect two identical devices, such as PC to PC or hub to hub.

72) What is a client/server?

A client/server is a type of network where one or more computers act as servers. The server provides a centralized repository for resources such as printers and files. The client is a workstation that accesses server.

73) Describe network.

A network is a connection between computers and peripherals used to transfer data. Network connections can be made using wired cables or wirelessly.

74) When moving a network card from one PC to another, does MAC address also move?

Yes, that's because MAC address is hardwired to NIC and not to PC. This also means that PC may have a different MAC address when network card is replaced with another one.

75) Aboutexplain clustering support

Clustering support refers to ability of a network operating system to connect multiple servers into failover groups. The main purpose of this is that in event of a server failure, next server in cluster will continue all processing.

76) In a network of two servers and twenty workstations, where is best place to install an antivirus program?

An antivirus program must be installed on all servers and workstations to ensure protection. This is because individual users can access any workstation and infect computer viruses by plugging in a removable hard drive or flash drive.

77) Describe Ethernet.

Ethernet is one of popular networking technologies in use today. It was developed in early 1970s and is based on specifications set out in IEEE. Ethernet is used in local area networks.

78) What are disadvantages of implementing a ring topology?

If one workstation on a network goes down, entire network can be lost. Another disadvantage is that when adjustments and reconfigurations need to be made in certain parts of network, entire network must also be temporarily shut down.

79) What is difference between CSMA/CD and CSMA/CA?

CSMA/CD or collision detection, which resend data frames every time a collision occurs. CSMA/CA, or collision avoidance, first transmits an intent to send before transmitting data.

80) What is SMTP?

SMTP is short for Simple Mail Transfer Protocol. The protocol handles all internal mail and provides necessary mail delivery services on top of TCP/IP protocol stack.

81) What is multicast routing?

Multicast routing is a form of targeted broadcast in which a message is sent to a selected group of users rather than all users on a subnet.

82) How important is encryption on Internet?

Encryption is process of converting information into code that cannot be read by user. It is then translated or decrypted back into its normal human-readable form using secret key or cipher. Encryption helps ensure that intercepted information remains unreadable, as users must have correct password or key.

83) How to organize and display an IP address?

IP addresses are displayed as a sequence of four decimal numbers separated by dots or dots. Another term for this arrangement is dotted decimal. Example:

84) Explain importance of certification.

Authentication is process of verifying a user's credentials before logging on to a network. This is usually done with a username and password. This provides a secure means of restricting access to unwanted maliciousnnikov to network.

85) What does tunnel mode mean?

This is a communication mode in which two communicating computers do not themselves use IPSec. Instead, gateway that connects local network to transit network creates a virtual tunnel that uses IPSec to secure all communications passing through it.

86) What technologies are used to establish a connection to global network?

Analogue connections - use regular phone lines; digital connections - use digital telephone lines; dial-up connections - use multiple sets of communication channels between sender and receiver to transfer data.

87) What is advantage of mesh topology?

If one link fails, another link is always available. Mesh topology is actually one of most resilient network topologies.

88) What are some common hardware-related problems that can come up when troubleshooting a computer network?

Most networks are made up of hardware. Problems in these areas can include hard drive failure, a faulty network card, or even hardware loading. Incorrect hardware configuration is also one of problems.

89) What can be done to fix signal attenuation problem?

A common way to solve this problem is to use repeaters and hubs, as this will help restore signal, preventing it from being lost. Checking that cables are connected correctly is also mandatory.

90) How does Dynamic Host Configuration Protocol help with network management?

Network ManagementInstead of visiting each client computer to configure a static IP address, administrators can use Dynamic Host Configuration Protocol to create pools of IP addresses, called ranges, that can be dynamically assigned to clients.

91) Explain an overview of network concepts?

Profiles are configuration options set for each user. For example, you can create profiles that group users into groups.

92) What is Sneakernet?

Sneakernet is considered to be earliest form of networking in which data is physically transferred using removable media (eg disks, tapes).

93) What is role of IEEE in computer networks?

The IEEE or Institute of Electrical and Electronics Engineers is an organization of engineers that publishes and administers standards for electrical and electronic equipment. This includes network devices, network interfaces, cables, and connectors.

94) What protocols exist at TCP/IP Internet layer?

This layer manages 4 protocols. These are ICMP, IGMP, IP and ARP.

95) Speaking of web, what are permissions?

Permissions are authorization rights to perform certain actions on network. Each user on network can beindividual permissions are assigned depending on what this user should allow.

96) What are basic requirements for creating a VLAN?

VLAN is required because there is only one broadcast domain at switch level, which means that whenever a new user connects, this information is propagated across network. VLANs on switches help create separate broadcast domains at switch level. It is used for security purposes.

97) What is IPv6?

IPv6 or Internet Protocol Version 6 was designed to replace IPv4. Currently, IPv4 is used to control Internet traffic, but IPv4 is already congested. IPv6 can overcome this limitation.

98) What is RSA algorithm?

RSA is short for Rivest-Shamir-Adleman Algorithm. It is currently most commonly used public key encryption algorithm.

99) What is mesh topology?

A mesh topology is a configuration in which each device is directly connected to all other devices on network. Therefore, each device is required to have at least two network connections.

100) What is maximum length of a 100Base-FX network segment?

The maximum allowable network segment length using 100Base-FX is 412 meters. The maximum length of entire network is 5 km.

Understanding basic knowledge from network guide (2)

We understand basic principles of modern network technologies from following 12 categories:

1. Network hierarchy

2. Seven-layer OSI network model

3. IP address

4. Subnet Mask and Network Separation


6. Routing Protocol

7. TCP/IP protocol

8. UDP protocol

9. DNS Protocol

10. NAT protocol

11. DHCP protocol

12. HTTP protocol

13. Example

The main content of learning a computer network is study of network protocols. A network protocol is a set of rules, standards, or conventions established for exchange of data on a computer network. Since data terminals of different users may use different character sets, they need to exchange data, which must be done according to a certain standard. A very vivid metaphor is our language, we have a huge land and a large population, and local language is also very rich, and there is a huge gap between dialects. The dialect in region A may not be acceptable to inhabitants of region B, so we need to establish a locale for communication of national names, which is role of our mandarin. Similarly, looking at world, standard language for communicating with foreign friends is English, so we must learn English diligently.

Computer network protocols are as diverse as our languages. However, ARPA Corporation launched widely known ARPANET network protocol from 1977 to1979 The main reason is that it launched well-known standard network protocol TCP/IP. TCP/IP has now become "universal language" of Internet

1. Network hierarchy

In order for computers from different manufacturers to communicate with each other to create a computer network of a larger scale, International Organization for Standardization (ISO) in 1978 proposed "Reference Model for Open Systems Interconnection". famous OSI/RM (Open System Interconnection/Reference Model).

It divides communication protocol of a computer network architecture into seven layers from bottom to top:

Physical layer

Data link layer

Network layer

Transport layer

Session layer

Presentation layer

Application layer

Among them, fourth level completes data transfer service, and top three levels face users.

In addition to standard seven-layer OSI model, common subdivisions of network layer include four-layer TCP/IP protocol and five-layer TCP/IP protocol, and correspondence between them

2. Seven-layer OSI network model

TCP/IP is undoubtedly underlying protocol of Internet. Without it, it is impossible to work on Internet. Any operation related to Internet is inseparable from TCP/IP protocol. Whether it's seven-layer OSI model, or four-layer and five-layer TCP/IP model, each layer needs its own exclusive protocol to do its work and communicate with upper and lower layers. Since seven-layer OSI model is standard division of network layers, we will introduce seven-layer OSI model from bottom up.

1) Physical layer

Activates, maintains, and deactivates mechanical, electrical, functional, and procedural features between communication endpoints. This layer provides a reliable physical medium for data transfer to upper layer protocol. Simply put, physical layer provides ability to transfer raw data on various physical media. The physical layer remembers two important device names, repeater (also called amplifier) ​​and hub.

2) Link layer

The link layer provides services to network layer based on services provided by physical layer. Its main service is to reliably transfer data from network layer to target neighbor network layer. To achieve this goal, data link must have a number of relevant functions, mainly including: how to combine data into data blocks, which is called a frame at data link layer, and frame represents data link. level; how to manage transmission of frames over a physical channel, including how to deal with transmission errors, how to adjust transmission rate to match with receiver; and ensure establishment and maintenancethe existence of data transmission channels between two network objects and release control. The link layer provides reliable transmission over unreliable physical media. The functions of this layer include: physical address addressing, data framing, flow control, data error detection, retransmission, etc.

Important information about data link layer:

1>The data link layer provides reliable data transfer for network layer;

2> The basic unit of data is frame;

3> Main protocol: Ethernet protocol;

4> Two important device names: bridge and switch.

3) Network layer

The purpose of network layer is to implement transparent communication between two end systems, and its specific functions include addressing and routing, connection establishment, service and termination, etc. It provides services in such a way that transport layer does not need to know transmission technology data and network switching. If you want to remember network layer as briefly as possible, it's "paths, routing, and logical addressing".

There are many protocols involved at network layer, including most important protocol, which is also core protocol of TCP/IP-IP protocol. The IP protocol is very simple and provides only unreliable, connectionless transfer services. The main functions of IP protocol are connectionless datagram transmission, datagram routing, and error control. There are ARP Address Resolution Protocol, RARP Reverse Address Resolution Protocol, ICMP Internet Message Protocol, and IGMP Internet Group Management Protocol, which are used together with IP protocol to implement its functions. We will briefly outline specific protocols in next section. Network layer key points:

1> The network layer is responsible for routing data packets between subnets. In addition, functions such as congestion control and Internet connectivity can also be implemented at network layer.

2> The basic unit of data is an IP datagram;

3> Basic conventions included:

IP Protocol (Internet Protocol, Internet Protocol);

ICMP protocol (Internet Control Message Protocol, Internet Control Message Protocol);

ARP protocol (Address Resolution Protocol, Address Resolution Protocol);

RARP protocol (Reverse Address Resolution Protocol, Reverse Address Resolution Protocol).

4> Important Equipment: Router.

4) Transport layer

The first pass-through level between hosts. The transport layer is responsible for segmenting upper layer data and providing end-to-end, reliable or unreliable transmission. In addition, transport layer also deals with end-to-end error control and flow control.

The goal of transport layer is to make best use of network resources in accordance withtvii with characteristics of communication subnet, provide functions of establishing, maintaining and canceling transmission connection between session layers of two end systems, and be responsible for end-to-end reliable data transmission. At this level, a protocol data unit for transmitting information is called a segment or message.

The network layer only forwards data packet sent by source node to destination node according to network address, while transport layer is responsible for reliable transfer of data to appropriate port.

Important points about network layer:

1> The transport layer is responsible for segmenting upper layer data and providing end-to-end, reliable or unreliable transmission, as well as end-to-end error control and flow control;

2> Major protocols included: TCP (Transmission Control Protocol, Transmission Control Protocol), UDP (User Datagram Protocol, User Datagram Protocol);

3> Important equipment: Gateway.

5) Session layer

The session layer manages session processes between hosts, that is, it is responsible for establishing, managing, and terminating sessions between processes. The session layer also implements data synchronization by inserting checkpoints into data.

6) Presentation layer

The presentation layer transforms top-level data or information to ensure that application layer information on one host can be understood by an application on another host. Presentation layer data transformation includes data encryption, compression, and format conversion.

7) Application layer

Provides an interface to operating system or network applications to access network services.

Key points of session layer, presentation layer and application layer:

1> The basic unit of data transmission is message;

2> Major protocols included: FTP (File Transfer Protocol), Telnet (Remote Login Protocol), DNS (Domain Name Resolution Protocol), SMTP (Mail Transfer Protocol), POP3 Protocol (Post Office Protocol), HTTP Protocol Hyper Text Transfer Protocol).

Three, IP address

1) Network address

An IP address consists of a network number (including a subnet number) and a host number. The host numbers of network address are 0, and network address represents entire network.

2) Broadcast Address

Broadcast addresses are commonly referred to as direct broadcast addresses to differentiate between limited broadcast addresses.

The host number of a broadcast address is just opposite of a network address. In a broadcast address, host number consists of ones. When a message is sent to a network broadcast address, all hosts on network can receive broadcast message.

3) Multicast Address

Class D addresses are multicast addresses.

Let'sfirst remember addresses A, B, C and D:

Class A addresses start from 0, first byte is used as network number, and address range is:; (modified @ 2016.05.31)

Class B addresses start with 10, first two bytes are used as network number, and address range is;

Class C addresses start at 110, first three bytes are used as network number, and address range is

Class D addresses start at 1110 and address range is Class D addresses are used as multicast addresses (one-to-many relationship).

Class E addresses start at 1111 and address range is to Class E addresses are reserved for future use.

Note. Only A, B, and C have network numbers and host numbers, and class D addresses and class E addresses do not have network numbers and host numbers.


This IP address is a restricted broadcast address. The difference between restricted broadcast address and general broadcast address (forward broadcast address) is that restricted broadcast address can only be used in LAN, and router will not use restricted broadcast address as destination address; address can be translated locally, and can also be translated across network segments. For example: after a forward broadcast datagram on host, another network segment can also receive datagram, if a restricted broadcast datagram is sent, it cannot be received.

Note. General broadcast addresses (forward broadcast addresses) can go through some routers (of course, not all routers), while limited broadcast addresses cannot go through routers.


It is often used to look up your own IP address. For example, in our RARP, BOOTP, and DHCP protocols, if a diskless machine with an unknown IP address wants to know its own IP address, it uses as destination address, sending an IP request packet to a server in local range (specifically , within range protected by each router).

6) Feedback Address

The loopback address is The loopback address specifies address of local machine. It is often used to check local machine. The most commonly used is

7) Private addresses of classes A, B and C

Private addresses are also called private addresses. They will not be used globally and only have a local value.

Class A private address:, range:

Class B private address:, range:

Class C private address:, range:

Four, subnet mask and network separation

With constant expansion of Internet applications, shortcomings of original IPv4, that is, too many network numbers and too few host numbers, are gradually being revealed, so host addresses that it can provide are getting smaller and smaller. In order to use NAT to allocate reserved addresses within an enterprise, high-level IP address is usually subdivided into several subnets for user groups of different sizes.

The main goal here is to efficiently use IP address in case of network segmentation. By taking higher part of host number as subnet number, subnet mask expands or shrinks from normal network bit boundaries to create more subnets for address class. But when more subnets are created, number of available host addresses on each subnet decreases than before.

What is a subnet mask?

The subnet mask indicates whether two IP addresses belong to same subnet, and it is also a 32-bit binary address, each of which is 1, which means a network bit, and 0 means a host bit. It is expressed as a dotted decimal notation, like an IP address. If two IP addresses have same bitwise AND subnet mask result, that means they belong to same subnet.

When calculating subnet mask, we must pay attention to reserved addresses in IP address, that is, address "0" and broadcast address, which refer to IP address when host address or network address is all "0" or "1" Addresses representing LAN and broadcast addresses are generally not counted.

Subnet mask calculation:

For an IP address that does not need to be subnetted, its subnet mask is very simple, that is, it can be written according to its definition: if class B IP address is, then it is not necessary to subnet, then subnet mask of this IP address is If it is a class C address, its subnet mask will be Other and so on, without details. The key point we want to present below is IP address and its high bits of host should be used as network number of divided subnet. The rest is host number of each subnet. How to mask each subnet at this time calculate .

Here is a summary of interview FAQs about subnet masks and network partitioning:

1) Use number of subnets to calculate

Before finding subnet mask, you must first determine number of subnets to be divided and number of hosts needed on each subnet.

(1) convert number of subnets to binary representation;

If you want to split class B IP address into 27 subnets: 27=11011;

(2) Get number of binary digits equal to N;

The binary code consists of five digits, N = 5

(3) Get class subnet mask of IP address and set first N bit of host address part to 1 to get subnet mask for subnet of IP address.

Set first 5 bits of host address of subnet mask of class B address to 1 to get

2) Use number of hosts to calculate

If you want to split class B IP address into multiple subnets, each with 700 hosts:

(1) convert number of hosts to binary representation;


(2) If number of hosts is less than or equal to 254 (note that two reserved IP addresses are removed), then get binary digit of host, which is N, where N<8 should be. If it is greater than 254, then N>8, which means that host address will take more than 8 bits;

The binary code consists of ten digits, N=10;

(3) Use to set all bits of host address of this type of IP address to 1, and then set all N bits to 0 at back, which is value of subnet mask.

Set all host addresses of class B subnet mask to 1 to get, and then set last 10 positions to 0 from back to front, that is: 11111111.11111111.11111100.00000000 , that is 255.255. 252.0. This is subnet mask of class B IP address, which should be divided into 700 hosts.

3) There is another type of question that requires you to plan subnet address and calculate subnet mask according to number of hosts on each network. This can also be calculated according to above principles.

For example, if there are 10 hosts on a subnet, then following IP address is required for that subnet:


Note. The first added unit refers to gateway address required for this network connection, and next two units refer to network address and broadcast address, respectively.

Since 13 is less than 16 (16 is 2 raised to 4th power), host bits are 4 bits. And 256-16=240 means subnet mask is

If there are 14 hosts on a subnet, many people often make mistake of still assigning a subnet with 16 address spaces, but forgetting to assign gateway addresses. This is wrong because 14+1+1+1=17, 17 is greater than 16, so we can only subnet 32 ​​addresses (32 equals 2 to 5th power). The current subnet mask is

Five, ARP/RARP protocol

The Address Resolution Protocol, namely ARP (Address Resolution Protocol), is a TCP/IP protocol that obtains physical addresses based on IP addresses. When a host sends information, it sends out an ARP request containing target IP address to all hosts on network and receives a response message to determine target's physical address; after receiving a responsemessage stores IP address and physical address in local ARP cache and keep it for a certain period of time, next request will directly query ARP cache to save resources. The address resolution protocol is based on mutual trust of each node in network. Nodes on network can send ARP response messages independently. When other nodes receive a response message, they will not authenticate message and record it. Enter local ARP cache; thus, an attacker can send a spoofed ARP response message to a specific host, so that information he sends cannot reach expected host or arrive at wrong host, which constitutes ARP spoofing. The ARP command can be used to query for a match between IP addresses and MAC addresses in local ARP cache, add or remove static matches, etc.

Example ARP workflow:

Host A's IP address is and MAC address is 0A-11-22-33-44-01;

Host B's IP address is and MAC address is 0A-11-22-33-44-02;

When host A wants to communicate with host B, address resolution protocol can translate IP address of host B ( into MAC address of host B. The procedure is as follows:

(1) Based on contents of routing table on node A, IP determines that IP address used to access node B is Host A then checks corresponding MAC address of Host B in its local ARP cache.address.

(2) If host A does not find a match in ARP cache, it requests hardware address, thus sending an ARP request frame to all hosts on local network. Both IP address and MAC address of source host A are included in ARP request. Each host on local network receives ARP request and checks if it matches its own IP address. If host finds that requested IP address does not match its own IP address, it will reject ARP request.

(3) Host B determines that IP address in ARP request matches its own IP address, then adds a mapping between Host A's IP address and MAC address to local ARP cache.

(4) Host B sends an ARP response message containing its MAC address directly back to host A.

(5) When host A receives an ARP reply message from host B, it updates ARP cache with mapping of host B's IP address and MAC address. The native cache has a time to live, after which above process repeats again. Once host B's MAC address is determined, host A can send IP traffic to host B.

The Reverse Address Resolution Protocol, RARP, performs opposite function of ARP. It converts physical address of a host on local network into an IP address. For example, if there is a host on local network that knows physical address, but not IP address, then it can send a broadcast request for its own IP address via RARP protocol, and then RARP-c is responsible for response.server.

RARP protocol workflow:

(1) send a local RARP broadcast to host, advertise its own MAC address in that broadcast, and ask any RARP server that receives this request to assign an IP address;

(2) Upon receiving a request, RARP server on LAN segment checks its RARP list to find an IP address that matches MAC address;

(3) If it exists, RARP server will send a response packet to original host and provide that IP address to other host for use;

(4) If it doesn't exist, RARP server won't respond to it;

(5) When a source host receives a response from a RARP server, it uses received IP address to communicate; if it did not receive any response information from RARP server, it means that initialization failed.

Six, routing protocol

Common routing protocols: RIP protocol, OSPF protocol.

RIP protocol: The bottom layer is Bellman-Ford algorithm. Its metric for route selection is number of hops. The maximum number of hops is 15. If number of hops is greater than 15, data packet will be dropped.

OSPF Protocol: Open Shortest Path First, bottom layer is Dijkstra's algorithm, which is a link-state routing protocol. The metrics for route selection are throughput and delay.


Seven, TCP/IP protocol

TCP/IP is simplest protocol on Internet and backbone of Internet. It consists of IP at network layer and TCP at transport layer. In layman's terms: TCP is responsible for detecting transmission problems and sends a signal whenever there is a problem, requiring a retransmission until all data has been safely and correctly transmitted to its destination. And IP is an indication of address for each network device on Internet.

The IP layer receives data packets from lower layer (the network interface layer, such as an Ethernet device driver) and sends data packets to a higher layer --- TCP or UDP layer; on contrary, IP layer also sends data packets from packets received at TCP or UDP layer are transmitted to lower layers. IP data packets are unreliable because IP does nothing to confirm whether data packets were sent in order or if they were corrupted. IP data packets contain address (source address) of host that sent it and host that received address (destination address). ).

TCP is a connection-oriented communication protocol. The connection is established through a three-way handshake. When connection is completed, connection should be dropped. Because TCP is connection-oriented, it can only be used for end-to-end purposes. connection to end. What TCP provides is a reliable data streaming service using "floor" technology.acknowledgment with retransmission” to ensure transmission reliability. TCP also uses a technique called "sliding window" for flow control. The so-called window actually represents receive bandwidth and is used to limit send rate of sender.

TCP message header format:

Note: seq: serial number of "sequence"; ack: acknowledgment number of "acknowledgment"; SYN: "sync" request synchronization flag; ACK: "acknowledge" acknowledgment flag; FIN: "Finally" completion flag.


The process of establishing a TCP connection: first, client sends a connection request message, and server segment responds with an ACK message after accepting connection and allocating resources for this connection. After receiving ACK message, client also sends an ACK message to server segment and allocates resources to establish a TCP connection.

The process of disconnecting a TCP connection. Suppose client initiates a release request, that is, sends a FIN message. After server receives a FIN message, it means "the client has no data to send to you", but if you have data left that hasn't been sent, you don't need to close Socket in a hurry, you can continue to send data. So you first send an ACK, "tell client that I have received your request, but I'm not ready yet, please keep waiting for my message." At this time, client enters FIN_WAIT state and continues to wait for a FIN message from server. When Server Side determines that data has been sent, it sends Client Side a FIN message, "Tell Client Side, well, I'm done sending data here, and I'm ready to close connection." After client receives FIN message, it knows connection can be closed, but it still doesn't trust network and is afraid server won't know to close it, so it goes into TIME_WAIT state after sending ACK. If server does not receive an ACK, it can restart Pass." After receiving an ACK on server side, "learn that connection can be dropped". After client side waits for 2MSL and still does not receive a response, this proves that server side was closed normally. Well, my client side can also close connection. Ok, TCP connection is closed!

Why are you waving your hand three times?

In case of only two handshakes, assuming client wants to connect to server, but because connection request datagram is lost in middle, client must resend it; at same time, server receives only a connection request, so connection can be established normally. However, sometimes client resubmits a request, not because datagram is lost, but because data transfer process can be blocked on a certain node due to a lot of concurrency in network, in this casethe server will receive two requests in a row, and keep waiting for two client requests to send data to it... The problem is that there is actually only one request on client side, but there are 2 responses on server side. In extreme cases, client side can resend multiple request data times As a result, server side ended up with more than N pending responses, causing a large waste of resources! Therefore, a "three-way handshake" is needed!

Why wave four times?

Imagine what if you are a client and want to terminate all connections to server? The first step is to stop sending data to server and wait for server to respond. But it's not over yet. Although you yourself are not sending data to server, since you previously established a peer connection, it also has initiative to send data to you at this time, so server must actively stop sending you data data, and wait for your confirmation. In fact, roughly speaking, this is ensuring full performance of contract between two parties!

Protocols using TCP: FTP (File Transfer Protocol), Telnet (Remote Login Protocol)), SMTP (Simple Mail Transfer Protocol), POP3 (relative to SMTP, used to receive mail), HTTP protocol, etc. .

Eight, UDP protocol

The UDP User Datagram Protocol is a connectionless communication protocol. The UDP data includes port number of destination port and source port number information. Since no connection is required for communication, broadcast transmission can be implemented. UDP communication does not require acknowledgment by receiver, which is an unreliable transmission, and packet loss may occur. Practical applications require programmers to verify programming.

UDP is on same level as TCP, but it doesn't care about packet order, errors, or retransmissions. Thus, UDP should not be used for connection-oriented services that use virtual circuits. UDP is primarily used for request-response services such as NFS. Compared to FTP or Telnet, these services need to exchange less information.

Each UDP message is divided into two parts: a UDP header and a UDP data area. The header consists of four 16-bit (2-byte) fields that respectively describe source port, destination port, message length, and message control value. The UDP header consists of 4 fields, each of which occupies 2 bytes, as follows:

(1) Source port number;

(2) Target port number;

(3) datagram length;

(4) Check value.

Protocols that use UDP include: TFTP (Simple File Transfer Protocol), SNMP (Simple Network Management Protocol), DNS (Domain Name Resolution Protocol), NFS, BOOTP.

Difference between TCP and UDP: TCP is a reliable stream of bytes, oconnection-oriented, while UDP is an unreliable, connectionless datagram service.

Nine, DNS protocol

DNS is an abbreviation for Domain Name System, which is used to name computers and network services organized in a hierarchy of domains, which can simply be understood as resolving URLs to IP addresses. A domain name consists of a sequence of words or abbreviations separated by dots. Each domain name corresponds to a unique IP address. There is a direct correspondence between domain names and IP addresses on Internet. DNS is a server for domain name resolution. DNS naming is used on TCP/IP networks such as Internet to locate computers and services by user-friendly names.


Ten, NAT protocol

NAT Network Address Translation is a wide area network (WAN) access technology. It is a translation technology that converts private (reserved) addresses to legal IP addresses. It is widely used in various types of Internet access methods and in various types of networks. The reason is very simple, NAT not only perfectly solves problem of lack of IP addresses, but also allows you to effectively avoid attacks from outside network, hide and protect computers inside network.

Eleven, DHCP protocol

Dynamic Host Configuration Protocol DHCP (Dynamic Host Configuration Protocol) is a LAN network protocol that uses UDP protocol to operate, it has two main purposes: automatic assignment of IP addresses to internal networks or network service providers, users or internal networks. Administrator as a means of centralized management of all computers.

Twelve, HTTP protocol

The Hypertext Transfer Protocol (HTTP, Hypertext Transfer Protocol) is most widely used network protocol on Internet. All WWW documents must conform to this standard.

What requests does HTTP protocol include?

GET: A request to read information identified by URL.

POST: Add information (such as comments) to server.

PUT: Save document to specified URL.

DELETE: Delete resource tagged with given URL.

In HTTP, difference between POST and GET

1) Receive - get data from server, and Post - send data to server.

2) Receive - Append a parameter data queue to URL pointed to by Action attribute of submitted form. The value corresponds to each field in form and is displayed in URL.

3) The amount of data transferred using Get is small and cannot exceed 2 KB; amount of data transferred using Post is relatively large and is generally considered unlimited by default.

4) According to HTTP specification, GET is used to retrieve information and must be safe and idempotent.

I. So-calledand safe means that operation is used to retrieve information, not change it. In other words, GET requests should generally not have side effects. That is, it only receives information about resources, like a database query, it will not change or add data and will not affect state of resources.

II. Idempotency means that multiple requests to same URL must return same result.

Thirteen example

The whole process after entering www.baidu.com browser

Now suppose that if we enter http://www.baidu.com in client (client) browser, and baidu.com is server (server) to be accessed, following detailed analysis of client's access to server operations performed by protocol:

1) The client browser resolves IP address www.baidu.com through DNS and finds path from client to server through this IP address. The client browser initiates an HTTP session at, then encapsulates data packet over TCP and injects it into network layer.

2) At transport layer of client, HTTP session request is divided into message segments and source and destination ports are added. Exchange, server returns appropriate request on port 5000 of client. Then use IP level IP address to find destination.

3) The network layer of client does not have to be associated with application or transport layer. The main thing is to determine how to get to server by looking at routing table. During this period, it can go through several routers. All this is done by router. Too many descriptions - nothing more than looking at routing table to determine which path to get to server.

4) Client link layer, packet is sent to router through link layer, MAC address of given IP address is looked up through neighbor agreement, and then an ARP request is sent to find destination. address, and an ARP request can be used after receiving a response. The IP packet that responds to exchange is now ready to be transmitted and IP packet is sent to server address.

Understanding basic knowledge from network guide (3)

1. Two ways to connect twisted pair:

EIA/TIA 568B standard and EIA/TIA 568A standard. The specific connection method looks like this:

T568A line sequence

green white green orange white blue blue white orange brown white brown

T568B line sequence

orange white orange green white blue blue white green brown white brown

Straight cable: Both ends are connected according to T568B line sequence standard.

Crossover cable: singleOne end is connected according to T568A line sequence, and other end is connected according to T568B line sequence.

When NIC is directly connected to 10M NIC, 100M NIC, a hub can be used, and crossover cable connection method should be used.

Switches and routers can now support T568B line sequence, so just remember 568B line sequence!

2. Optical transceiver module

The light module consists of optoelectronic devices, functional circuits and optical interfaces, etc. Optoelectronic devices consist of two parts: transmitting and receiving. Simply put, function of optical module is photoelectric conversion. The transmitting end converts electrical signal into optical, after transmission through optical fiber, receiving end converts optical signal into electrical.

Install network card on computer and configure; connect power to transceiver and operate in strict accordance with instructions; connect computer and transceiver twisted pair, twisted pair This must be a crossover connection, use an optical jumper to connect two transceivers. If transceiver is single mode, jumper must also be single mode. When optical jumper is connected, one end is connected to RX and other end is connected to TX, so a cross connection. However, many optical modules now have control functions, and both crossover and straight-through cables can be used.

3. Network Switch

A network switch is a network expansion device that can provide more connection ports on a subnet to connect more computers. With development of communication industry and informatization of national economy, network switch market shows a steady upward trend. It has characteristics of high cost efficiency, high flexibility, relative simplicity, ease of implementation, and so on. Thus, Ethernet technology has become most important LAN network technology today, and network switches have become most popular switches.

4. Ping Command

Ping is a command on Windows, Unix, and Linux systems. Ping is also a communication protocol and is part of TCP/IP protocol. Use "ping" command to check if network is connected, which can help us analyze and identify network failures.

Under normal circumstances, users can use a series of ping commands to find out what problem is or check network.

A typical detection sequence and corresponding possible errors are shown below:

① ping

If test is successful, it means that network card, TCP/IP protocol setting, IP address, and subnet mask settings are OK. If test failsn, it means that there was a problem installing or configuring TCP/IP.

② Ping local IP address

If test fails, it means that there is a problem with local configuration or installation, and network and communications equipment should be tested, verified, and fixed.

③ Ping other IPs on local network

If test is successful, NICs and LAN bearers are working properly. But if you get 0 echo replies, then subnet mask is wrong, or network card is not properly configured, or there is a cabling problem.

④ check gateway IP address

If this command is answered correctly, gateway router on local network is up and able to respond.

⑤ check remote IP address

If a valid response is received, default gateway has been used successfully. For dial-up Internet users, this means that they can successfully access Internet (but do not rule out that DNS provider may have problems).

⑥ ping localhost

localhost is a network reserved system name that is an alias for and every computer should be able to resolve this name to this address. Otherwise, it indicates a problem in hosts file (/Windows/host).

⑦ ping www.qq.com (Tencent's official domain name)

To Ping this domain name, computer must first resolve domain name to an IP address, usually through a DNS server. If an error occurs here, it means that configuration of local DNS server IP address is incorrect or DNS server it is accessing is faulty

If all of ping commands listed above work correctly, then computer should basically be able to communicate locally and remotely.