ccna 200-301 network simulator toolstation near

ccna 200-301 network simulator toolstation near.In the second generation computer network, multiple computers form an organic whole through the communication subnet, which is both decentralized and unified, thus making the whole system performance much higher. The load of the original single host can be distributed to individual computers throughout the network, making the network system more responsive, and in such a system, the failure of a single computer does not result in a complete breakdown of the entire network system. In a network, computers communicating with each other must work in a highly coordinated fashion, and this "coordination" is quite complex. To reduce the complexity of network design, a hierarchical model was proposed by experts back when ARPANET was first designed. A hierarchical design approach can transform a large and complex problem into a number of smaller and more manageable sub-problems.Having a network architecture makes it very easy for the various machines and network devices produced by a company to be connected together. However, because the network architecture varies from company to company, the networks of different companies cannot be interconnected. In response to this situation, the International Standard Organization (ISO) set up a special body in 1977 to study the problem and soon afterwards proposed a standard framework for interconnecting computers, the Open System Interconnection/Reference Model (OSIRM). The Open System Interconnection/Reference Model (OSI/RM), or OSh for short, is an open architecture that specifies a network in seven layers and defines the functions of each layer. The emergence of the OSI reference model means that computer networks have evolved into the third generation. After the OSI reference model out, the development of the network has been the road of standardization, and the greatest embodiment of network standardization is the rapid development of the Internet. Now the Internet has become the world's largest international computer Internet. The Internet follows the TCP/IP reference model, and because TCP/IP still uses a hierarchical model, the Internet is still a third-generation computer network. Nowadays, computer networks have been gradually systematized, scientific and engineered from architecture to practical technology.In order for two computers to communicate, they must be made to use the same rules for information exchange. We call the set of rules used in computer networks to specify the format of information, and how to send and receive it, a network protocol (Network15 AProtocol) or CommunicationProtocol To reduce the complexity of network protocol design, rather than designing a single, huge protocol that specifies the complete details for all forms of communication, network designers use the approach of dividing the communication problem into many smaller problems and then designing a separate protocol for each smaller problem. This makes it easier to design, analyze, code, and test each protocol, just as we do with programming, by writing "procedures" and "functions" for easy invocation, to modularize, simplify, and make a complex program independent. The hierarchical model is a design approach for developing network protocols. Essentially, the hierarchical model describes a way to divide a communication problem into smaller problems (called layers), each of which corresponds to a single layer.To reduce the complexity of network design, the vast majority of networks use a layered design approach. The so-called hierarchical design approach is to decompose the overall functionality of the network into functional layers according to the flow of information, using the same protocols between equivalent functional layers on different machines, and passing information between adjacent functional layers on the same machine through interfaces.To facilitate understanding of the concepts of interfaces and protocols, the following is an example of a postal communication system. When people normally write letters, there is a protocol, which is the format and content of the letter. First of all, the letter must be written in a language and style that both parties understand, beginning with the title of the other party and ending with a closing note, etc. This way, when the other party receives the letter, he or she can read the contents and know who wrote it, when it was written, etc. After the letter is written, the envelope must be put in and given to the post office to send, so that there is also an agreement between the sender and the post office, which is to specify the envelope writing and postage stamps. In China letters must be sent with the address and name of the recipient first and then the address and name of the sender. After the post office receives the letter, it first sorts and classifies the letter, and then delivers it to the relevant transport department for transport, such as airmail to civil aviation, flat mail to railway transport or road transport, etc. At this point, the post office and the transport department also have an agreement, such as the place and time of arrival, the form of the package, etc. The letter is delivered to the destination and then the opposite process is carried out and finally the letter is delivered to the recipient, who can read the letter only according to the agreed format. As shown in Figure 2.2.1, there are three main subsystems involved in the whole process, i.e., user subsystem, postal subsystem and transportation subsystem. As can be seen from the above example, the various conventions are designed to achieve the goal of delivering the letter from a source to a destination, which means that they arise as a result of the flow of information. These engagements can be divided into those between equivalent agencies, such as those between users, those between post offices and transportdepartment, and those between different agencies, such as those between users and post offices, and those between post offices and transport departments.Although the two subscribers, the two post offices and the two transport departments are located in A and B, they all correspond to the same institution respectively and belong to the same subInter-user agreement