"Description": a graph composed of nodes and lines. Represents the research object and its relationship. Sometimes a line with an arrow indicates that there is a certain order relationship from one node to another. The numerical values marked next to nodes or lines are called point weights or line weights, and sometimes they are not marked with numbers. In mathematical language, the network is a graph, which is generally considered as a weighted graph. In addition to the mathematical definition, the network also has a specific physical meaning, that is, the network is a model abstracted from a certain kind of practical problems, and what kind of network is customarily called, such as switching network, transmission network, communication network, planning network and so on. In a word, the network is a model abstracted from similar problems and expressed and studied by graph theory in mathematics.

Network concept:

A system that connects several autonomous computer systems distributed in different places with communication lines and communication equipment and shares hardware, software and data resources according to the same network protocol.

Four elements:

1, communication lines and communication equipment

2. Computers with independent functions

3, network software software support

4. Realize data communication and resource sharing.

The development of computer network

Remote terminal online stage

Computer network stage (LAN)

Computer network interconnection stage (WAN, Internet)

Information superhighway stage (high speed, multi-service, large data volume)

Computer network development

First Generation: Remote Terminal Connection

In the early 1960s.

Terminal-oriented computer network: the host is the center and controller of the network. Terminals (keyboards and monitors) are distributed everywhere and connected to the host computer. Users use remote hosts through local terminals.

Communication between terminals and hosts is only provided, and communication between subnets is impossible.

The second generation: computer network stage (LAN)

The mid-1960s

Multiple hosts are interconnected to realize communication between computers.

Comprise a communication subnet and a user resource subnet.

End users can access the hardware and software resources of local hosts and all hosts on the communication subnet.

Circuit switching and packet switching

The third generation: computer network interconnection stage (wide area network, Internet)

1977, the International Organization for Standardization (ISO) formulated the basic reference model (OSI/RM) for interconnection between computers produced by different manufacturers.

The birth of TCP/IP protocol

The fourth generation: information superhighway (high-speed, multi-service, large data volume)

Broadband integrated services digital network: information superhighway

ATM technology, ISDN, gigabit Ethernet

Interactivity: online TV on demand, video conference, videophone, online shopping, online banking, online library, etc.

High speed and visualization

The development history of computer network

From 65438 to 0969, ARPA- Advanced Research Projects Agency of U.S. Department of Defense began to build a network called ARPAnet, which connected several military and research computer mainframes in the United States. At first, ARPANET only connected four hosts. As far as military requirements are concerned, it is under the protection of high-level secrets of the US Department of Defense. Technically, it does not have the conditions for external promotion.

1983, ARPA and the us department of defense communications bureau successfully developed the TCP/IP protocol for heterogeneous networks. The University of California, Berkeley took this protocol as a part of its BSD UNIX, which made it popular in the society and gave birth to the real Internet.

From 65438 to 0986, the National Science Foundation (NSF) established NSFnet WAN on the basis of five supercomputers for scientific research and education services by using the TCP/IP communication protocol developed by ARPAnet. ?

Before the 1990s, the use of the Internet was always confined to research and academic fields. Commercial organizations' access to the Internet is always troubled by laws or traditional problems of one kind or another. In fact, government agencies such as the National Science Foundation, which once funded the construction of the Internet, are not interested in commercial activities on the Internet. 199 1 year, three companies in the United States operated their own CERFnet, PSInet and Alternet networks, which can provide customers with internet networking services to some extent. They set up the Commercial Internet Association (CIEA), announcing that users can use their Internet subnets for any commercial purpose.

Popularization of the Internet At present, the Internet has contacted 160 countries and regions, more than 40,000 subnets, more than 5 million computer hosts and more than 40 million direct users, making it the most abundant computer network in the world. Internet is considered as the embryonic form of the future global information superhighway.

Network classification

By coverage:

Local area network LAN (the range of action is usually several meters to dozens of kilometers)

Man (between Wan He Lan)

Wide area network WAN (the scope of action is generally tens to thousands of kilometers)

Classification by topological structure

Bus type

toroidal

star

reticulation

According to the way of information exchange:

circuit switching

Message exchange

packet switching technology

A network can consist of two computers, or it can have thousands of computers and users in the same building. We usually call such a network a local area network, which extends to a larger area, such as the whole city or even the whole country. Such a network is called a wide area network. Of course, if you want to divide it carefully, there can also be MAN (Metropolitan Area Network) and CAN (Metropolitan Area Network), and these networks need to be maintained by special managers.

And the Internet, which we contact most often, is composed of countless local area networks and wide area networks. Internet only provides the connection between them, and there is no special person to manage it (except for maintaining the connection and formulating the use standard). It can be said that the Internet is the freest and most unregulated place. There is no distinction between national boundaries and races on the Internet. As long as it is connected, the computer on the other side of the world is actually no different from your roommate's computer.

Because we still use the local area network most often (even if we connect to the Internet from home, we actually connect to the ISP's local area network first), so here we mainly discuss the local area network. Local area network can be said to be the most basic unit in many networks. When you have a certain understanding of the local area network, it will be easier to understand the wide area network and the Internet, but you just need to know more and more complicated communication means.

Internet? Intranet? External network?

Friends who have been exposed to the internet should have heard the above nouns more or less, right? However, do you know the difference between them and how to define them?

In fact, the earliest term should be Internet. Later, people introduced the concept and skills of Internet into the internal private network, which can be an independent local area network or a dedicated wide area network, so it was called Intranet. The biggest difference between the two is openness. The Internet is open and belongs to no one. As long as you can connect, you belong to one of them, and you will also get the open resources on it. Relatively speaking, the intranet is exclusive and non-open. It often exists on the private network, but its structure, service mode and design all refer to the model of the Internet.

Internet and Intranet

As for Extranet, it can be regarded as an extended concept of Intranet. Since the intranet refers to the intranet department, the extranet refers to the extranet. Extranet is usually a network where enterprises connect to the Internet to provide services to the public. However, this is not simply defined by physical or logical location, but mainly by the form and function of the connection. For example, an external network, if it is simply connected to our external network or internal network through the network, is only an irrelevant external individual; But if we use VPN or other forms of trust to connect each other, then the other party can also belong to the external network or the Internet.

In the history of human development, the emergence and development of electronic computers has a long history. However, the computer-based Internet, I don't know when it started, quietly entered and integrated into our lives. With the development of modern society, the dependence on the Internet has reached an unprecedented level.

Network has brought a powerful way of information dissemination, greatly shortened the time of information release and reception, and avoided many unnecessary waste of resources.

Since the network began to spread slowly, major companies have established their own web pages and websites on the Internet to introduce their own situation and publicize their products. More companies openly solicit orders online to better promote their products. There are more and more similar websites, opening many different kinds of online shopping malls, and flaunting their products as cheaper and more cost-effective than usual. In an instant, information spread widely among netizens, resulting in a full exchange of supply and demand information. As long as people enter the goods they want online and search, they will present a lot of supplier information. Suppliers can also use the information on the Internet to inquire which consumers have the greatest consumption capacity for their goods and statistical data, so as to better publicize them. People can do business online without leaving home, which greatly promotes the situation of both supply and demand in the market.

In today's highly developed information industry, people's demand for information is increasing. Internet has become the best media. For students, many large-scale exam registration and results inquiry can be completed online, without making a busy and expensive voice call as before. Details of the national unified civil service examination in the past two years have also been published online. Candidates can see a series of details such as recruitment department, number of applicants, positions and requirements at a glance as long as they open the recruitment website in their region. It is also clearly displayed on the internet together with the materials and registration places that need to be prepared for the application, so that candidates can understand the situation very conveniently and easily. For units that publish information, it is no longer necessary to send personnel to promote it from place to place; For candidates who want to know the situation, they don't have to go all the way to the designated place to get the enrollment brochure and understand the enrollment situation. Both sides have benefited a lot from the spread of network information.

The network has greatly narrowed the distance between people.

As we all know, games on the Internet are constantly developing and expanding, and there are more and more kinds of games. Take OICQ as an example. Because of the distance between time and space, many friends can't always keep in touch with reality. In these cases, QQ provides a good contact way for friends. In QQ, you can not only keep in touch with old friends, but also make many new friends. In the short chat and communication, the spiritual distance between people has been narrowed. In other different online games, people can also get to know each other and become good friends through mutual cooperation and help in the game. People can also write to each other online by email and tell each other how they are doing.

In the last two months of SARS, the epidemic situation in different regions was different. In Hong Kong and Beijing, where the epidemic is serious, a large number of primary and secondary school students are suspended. During the period when students are out of school, the education department only uses the network to open an "air classroom" on the Internet to provide students with basic academic questions and exercises, so that students can study as usual at home. The novelty of learning methods attracts students' attention and improves their interest in learning.

Similar distance education is also used in many other teaching. For example, when we were in senior three, we couldn't go back to school to take the rest of the courses because some students went out to work. However, whether you are working at school or outside, you must take the graduation exam. In order to facilitate the students who are absent from class, the teacher puts notes on the internet to facilitate students to study by themselves.

Not only education and study, but also trade meetings can be held online. The word "network conference" is widely popular on the Internet. Merchants and companies from different countries and regions can hold online meetings through the network to coordinate and discuss important business issues, which saves a series of tedious procedural steps such as convening participants and agreeing on the time and place of the meeting, and saves a lot of manpower and material resources.

However, while the network brings us various benefits, it also brings us many disadvantages.

First of all, the rapid development of the Internet has brought loopholes to unscrupulous businessmen. There are many bad websites such as pornography, violence and gambling. In order to make huge profits, these unscrupulous businessmen keep spreading pornographic profiteering information on the website, regardless of how harmful the information on the website is to the ideological health of teenagers. What's more, making different pornographic films requires the audience to pay before viewing them. In addition to pornographic profiteering websites, some websites have telephone charging systems. Once visitors open their web pages, they will be charged extremely expensive and unreasonable long-distance telephone charges, which will cause huge economic losses to visitors. There are many intermediary websites on the Internet that introduce jobs, recruit friends and marriage, but there are also many websites that cheat money and fail to fulfill their obligations. Netizens will step into these elaborate traps if they are not careful.

Secondly, the internet has brought many disputes between people. Because the government's policy on the Internet is not perfect, many questions about intellectual property rights are constantly emerging. For example, after some articles were published on the internet, they were copied and revised many times, and then they were re-published after being taken for themselves, which seriously violated the copyright of the original author. However, due to the complexity and uncertainty of the network, the source of plagiarism is often not found. For another example, the accounts we have in online games are sometimes illegally stolen, which is actually a criminal responsibility, but we often let things go away because we can't find a thief. The account of the game is still a trivial matter. If important business information is stolen and relevant business secrets are leaked, the stolen company will suffer serious losses. If the theft is a state secret, the consequences are even more unimaginable. The military and national defense system of the whole country may be greatly damaged, and even immeasurable manpower and material resources are needed to re-establish military data.

The Internet has not only brought invisible harm to people's minds, but also caused great harm to the health of many netizens. On TV, we have seen such reports many times: a teenager suddenly became paralyzed and unable to move because he played games online for too long, and in severe cases he went into shock and coma. Due to the radiation of the computer to the human body, if people sit in front of the computer for a long time without activity, it is very easy to cause all kinds of unpredictable physical injuries. The most obvious is the damage to vision. According to statistics, people who have been engaged in computer work for a long time have much worse overall vision than those who rarely touch computers.

The network has brought immeasurable convenience, opportunities and wealth to people, and also brought many unexpected potential crises to people. The internet is really a place that people love and hate.