Definition

Computer Network refers to the software and hardware system that realizes communication between computers, in a broad sense, copying data between two computers with a disk can also be considered as a special network. Its more specific definition is “a system consisting of interconnecting multiple computers through a data communication line for the purpose of sharing resources”, the shared resources include hardware devices, software or data in a computer network.

Network topology and classification

In computer network, the connection mode of Node computer or terminal is called the network topology structure, which affects the design, function, reliability and communication cost of network. The common topology structure has the following types: (Fig. 13-1)

• Star topology;

• Bus topology;

• Ring topology;

• Tree Topology;

• Complete connection structure;

• Intersection ring;

• Irregular topology and so on.

Figure 13-1: Network topology (A. S. Tanenbaum)

(1)bus structure, (2) ring structure, (3) star structure, (4) tree structure,

(5)fully connected, (6) intersecting rings, (7) irregular topologies

There are many kinds of computer networks with different performances, different classifications can be used according to different principles, the most common ones are LAN-Local Area Network and MAN-based network according to the size of their spatial distribution, metropolitan Area Network) and WAN-Wide Area Network.

1)LAN

It refers to a computer network established in a relatively small area, such as an office building, a campus, a company, etc., with a short communication distance, a fast transmission rate, and a low bit error rate. For LAN, the main topology is bus structure and ring structure, the former, such as IEEE 802.3, is usually called Ethernet Circuit: sup: TM Circuit (Ethernet), while the latter, such as IEEE802.5, is IBM Token Ring. For LAN, the main topology is bus structure and ring structure, the former, such as IEEE 802.3, is usually called Ethernet Circuit: sup: TM Circuit (Ethernet), while the latter, such as IEEE802.5, is IBM Token Ring.

2)Metropolitan area network

Basically a large local area network with a similar topology, it can cover a city and share information.

3)WAN

The most fundamental feature of the WAN is that it has a wide range of machines and can realize a wide range of information sharing, in general terms, the Internet belongs to the WAN category. The WAN can adopt a variety of topologies, such as stars, rings, trees, full connections, intersecting rings, and even irregular topologies.

In the above network types, the nodes are connected by cables, in recent years, the Wireless Network has been developed rapidly, and the wireless network can be used for mobile computing.

Internet development history

The predecessor of the Internet was the ARPA (Advanced Research Project Agency) network, which was designed by the US Department of Defense in 1965. After careful planning, in 1969, computers distributed in four universities (UCLA, UCSB, SRI, UTAH) began to exchange data with each other, marking the birth of ARPAnet. Later, especially in 1983, the TCP/IP protocol was officially applied after ARPAnet, its access users grew rapidly.

Due to the military background of ARPAnet, it is inconvenient for more convenient connection and access, therefore, NSF (National Science Foundation) established NSFnet, which is superior in management and speed to ARPAnet, by 1990, many users switched from ARPAnet to NSFnet, later, NSFnet and ARPAnet were interconnected, and their users grew exponentially. The networking of many regional (including Canada, Europe, Oceania, etc.) marks the final formation of the Internet.

Internet-provided application services

Traditionally, the internet has four main applications:

• Electronic Mail;

• News;

• Remote login;

• File transfer.

Until the early 1980s, the Internet was limited to colleges, governments, etc., but the emergence of the World Wide Web (WWW) changed all of them, the WWW enabled a site to create pages that included text, images, sounds, and even video, these pages are written in HTML-Hyper Text Markup Language, with links to other pages embedded. The WWW provides great convenience for users to query, retrieve, and browse various information published on the Internet, bringing a large number of non-college users to the Internet. In fact, the rapid development of the Internet is inseparable from WWW applications, so that when it comes to the Internet today, it is often referred to as WWW services, the WWW and related technologies will be described in more detail later.

Other Internet applications include Gopher, WAIS and so on.

Protocol and TCP/IP

In computer networks, in order to enable the correct transfer of information between connected computers or terminals, there must be a set of conventions for information transmission order, information format, and content, called computer network communication protocols.

In the network protocol, the International Organization for Standardization (ISO) develops and develops the Open System Interconnect Reference Model (OSI) to develop standards for data communication protocols, although the OSI model is not really applied in practice, however, the concepts and vocabulary provided by it are widely used and recognized by the computer network community. The OSI reference model includes seven protocol layers to define the protocol functions of data communication, each layer is relatively independent and performs some functions in the data transmission process.

At present, due to the development of the Internet, TCP/IP, as its basic protocol, is more and more widely used. TCP/IP narrowly refers to two protocols and transmission control protocol (TCP, Transformation Control Protocol) and Internet Protocol, broadly speaking, it refers to the TCP/IP reference model composed of several Internet-related protocols, or called TCP/IP protocol stack (Protocol Stack), TCP/IP model is similar to OSI reference model, but it is relatively simple, without presentation layer and session layer. TCP and IP are two important protocols, IP is the network layer protocol, and TCP is located in the transport layer.

TCP/IP application layer includes all high-level application protocols, such as TELNET, FTP, File Transfer Protocol, SMTP, Simple Mail Transfer Protocol and PO3-Post Office Protocol, DNS, Domain Name Service, and HTTP, HyperText Transfer Protocol, which is used to transfer pages on the Web.

The structure of OSI reference model and its corresponding relationship with TCP/IP reference model are shown in Figure 13-2:

Figure 13-2: OSI Reference Model and TCP/IP Reference Model

Internet address, domain name

In order to be able to transmit information over the Internet, there must be a way to identify the computer. The solution is to give each computer on the Internet a unique 32-bit address called an IP address, which is used by the 32-bit address. The decimal number of the number is divided. Each byte is a decimal value, and its value ranges from 0 to 255. Thus, the IP address appears as the following form: 202.112.7.12 (IP address of Peking University WWW server). On the Internet, communication between computers is done through IP addresses. Due to the rapid development of the Internet, more and more computers are connected, and the existing 32-bit IP address is no longer sufficient for further development, therefore, ITEF (Internet Engineering Task Force) began to develop the updated IP protocol standard, called Ipv6, according to which the IP address will be identified by 16 bytes.

Although the communication between computers is realized by using their respective IP addresses, people do not want to always use hard-to-remember numbers in the process of using them, but want to use more convenient text names. In this way, each host on the network usually has a name in addition to the IP address. This name is called a domain name (Domail Name) and is the same as an IP address. This name system used on the Internet is called the Domain Name System (DNS). DNS is a hierarchical naming system that manages the names and addresses of all hosts connected to the Internet through a database. Domain names are strings separated by dots, such as PKU.EDU.CN and OPENGIS.ORG. Usually the rightmost string is called a top-level domain. There are two types of top-level domains, namely the top-level domain and the national top-level domain, the former has com(commercial), edu(educational), gov(government agency), int(specific international organization), mil(army system), net(network provider), org(non-profit organization), etc., top-level domain names are applied to sites in the United States; the latter uses two English letters to represent a country and region, such as cn(China), ca(Canada), jp(Japan), and so on.

WWW, HTML and URL

As described earlier, the advent of the World Wide Web (WWW) has a epoch-making significance for the Internet, making the application of the Internet professional. WWW, often referred to as the Web or 3W, is a collection of millions of documents stored in Internet computers around the world. It is the world’s largest electronic information warehouse. In a WWW application, a Web document is stored on one or more computers, called a Web Server, and a user uses a browser to access a Web site.

The first version of the Web was first launched in Geneva, Switzerland in January 1992, researchers can access Web information from the CERN (European Institute of Particle Physics) site. The real popularity of the Web should be attributed to Mosaic, the first graphic-based browser designed and written by NCSA (National Center for Supercomputing Applications), which is easy to operate and user-friendly, so that people can easily access Web information. Currently popular browsers are Microsoft Internet Explorer and Netscape Navigator.

On the Web site, documents are written in HTML, commonly referred to as Hyper Text, “Super” means that the document contains hyperlinks to other documents (Hyper Link), so that users can use keywords when reading. Jump to other documents. The concept of Hypermedia was born by allowing multimedia data such as music and images to be added to hypertext. HTML is a markup language that is independent of the system platform and application, which sets the structure or format of a Web document with special markup and reference characters, and has special HTML elements (such as the anchor element Anchor) to define hyperlinks, at the same time, you can use HTML form elements to get information from users. When a user visits a Web site, the request and information are sent between the browser and the Web server via Hypertext Transfer Protocol (HTTP).

When browsing the Web, you will first encounter the concept of URL (Uniform Resource Locator), which is the Internet address of Web documents, and its relationship with documents is similar to that of catalogues and text in books. The syntax of URLs is quite simple, consisting of two parts, such as:

: (: )

An HTTP URL is as follows:

The host part is composed of the server’s address (IP address or domain name), file path and file name; the port part is HTTP port number, which defaults to 80 and can be ignored; and the search part part part is often the request parameter sent by the browser to the server. Here is an example of an HTTP URL:

In addition to the Web, FTP, Gopher and others also use URLs to locate resources.

Intranet

In the second half of the 1990s, with the development of the Internet, the new concept of intranet (Intranet) emerged, in short, the intranet is the enterprise’s own internal network, but this network also uses Internet-based tools, such as Email, Web browser and file transfer, etc., thus enabling an enterprise to fully enjoy the advantages of the Internet, such as information sharing and management, real-time communication and collaboration, the use of distributed databases and so on.