Unit 9 The Internet

nullUnit 9 The InternetUnit 9 The InternetSection A The Internetnull The most notable example of an internet is the Internet (note the uppercaseⅠ), which originated from research projects going back to the early 1960s. null The goal was to develop the ability to link a variety of computer networks so that they could function as a connected system that would not be disrupted by local disasters. null Most of this original work was sponsored by the U.S. government through the Defense Advanced Research Projects Agency (DARPA- pronounced DAR-pa). Over the years, the development of the Internet shifted from a defense project to an academic research project, and today it is largely a commercial undertaking that links a worldwide combination of WANs, MANs, and LANs involving millions of computers.Ⅰ. Internet architectureⅠ. Internet architecture Conceptually, the Internet can be viewed as a collection of domains, each of which consists of an network or a relatively small internet operated by a single organization such as a university, company, or government agency. Each domain is an autonomous system that can be configured as the local authority desires. It might consist of a single computer or a complex internet consisting or many LANs, MANs, and even WANs.null The establishment of domains is overseen by the Internet Corporation for Assigned Names and Numbers (ICANN), which is a nonprofit corporation established to coordinate the naming of domains and the assignment of Internet addresses. To establish a domain on the Internet, the domain must first be registered via one of the companies, called registrars, that have been delegated for this purpose by ICANN.null Once a domain has been registered, it can be attached to the existing Internet by means of a router that connects one of the networks in the domain to a network already in the Internet. This particular router is often referred to as the domain’s gateway, in that it represents the domain’s gate to the rest of the Internet. null From the point of view of a single domain, the portion of the Internet lying outside of its gateway is sometimes called the cloud, in reference to the fact that the structure of the Internet outside the domain’s gateway is out of the domain’s control and is of minimal concern to the operation of the domain itself. null Any message being transmitted to a destination within the domain is handled within the domain; any message being transmitted to an destination outside the domain is directed toward the gateway where it is sent out into the cloud. null If one “stood” at a domain’s gateway and “looked out ” into the cloud, one could find a variety of structures. Indeed, the Internet has grown in a somewhat haphazard way as various domains have found points at which to connect to the cloud. One popular structure, however, is for the gateways of a number of domains to be connected to form a regional network of gateways. null For instance, a group of universities could choose to pool their resources to construct such a network. In turn, this regional network would be connected to a more global network to which other regional networks attach. In this manner, that portion of the cloud takes on a hierarchical structure.null区域 0.0.0.1区域 0.0.0.3主干区域 0.0.0.0R9R7R6R5R4R3R2R1网 8网 6网 3网 2网 1网 7区域 0.0.0.2网 4网 5R8The Internet cloudⅡ. Connecting to the InternetⅡ. Connecting to the Internet To simplify the process of connecting to the Internet, numerous companies, called Internet service providers (ISPs), allow customers to connect their domains to the Internet via the ISP’s equipment or to become a part of a domain already established by the ISP. null Perhaps the least expensive connections to an ISP are obtained through temporary telephone links called dial-up connections. Using this approach, an individual connects his or her computer to the local telephone line and executes a software package that places a call to a computer at the ISP. At this point the ISP provides Internet access for the duration of the telephone call.null This temporary connection model is popular (and, in fact, expanding) in the cell-phone/hand-held computer arena. However, in fixed location applications it is rapidly giving way to permanent connections through higher bandwidth systems such as DSL, cable television lines, and satellite links. null These technologies have greatly reduced the expense of broadband (high-capacity) Internet access (which used to be available only to large organizations that could afford the expense of dedicated Internet connections), making today’s multimedia applications accessible to home users.null Another development taking place at the household level is that home computer systems are expanding from single computer installations to multicomputer networks, usually implemented as Ethernet bus networks. null These networks gain access to the Internet by means of a router within the home that is permanently connected to an ISP via DSL, a cable television line, or a satellite link, making the home system a local network within a domain established by the ISP. null The connection from the home’s router to the local computers is often wireless. That is, each computer communicates with the router by means of a small transmitter/receiver with a range of approximately 100 feet. null Such wireless local networks are popular outside the home as well where they are used to create hotspots in which any wireless-enabled computer (laptop or hand held) that comes within range can access the Internet. Examples include office buildings, “cyber cafes”,public plaza, and entire cities.null In closing we should note that ISPs provide much more than mere access to the Internet. In many cases they are the companies that construct and maintain the Internet infrastructure. null Moreover, their services generally include a wide array of both hardware and software support for their customers. In fact, and ISP might provide a significant portion of the computing resources (such as data storage and data processing services) required by a client.Ⅲ. Internet AddressingⅢ. Internet Addressing An internet must be associated with an internet-wide addressing system that assigns and identifying address to each computer in the system. In the Internet theses addresses are known as IP addresses. null Each IP address is a pattern of 32 bits, although plans are currently underway to increase this to 128 bits. Each 32-bit address consists of two parts: a pattern identifying the domain in which the computer resides and a pattern identifying the particular computer within the domain. null The part of the address identifying the domain, the network identifier, is assigned under the authority of ICANN at the time the domain is registered. Thus it is by means of this registration process that each domain in the Internet is assured of having a unique network identifier. null The portion of the address that identifies a particular computer within a domain is called the host address. The host address is assigned by the domain’s local authority-usually a person having a job title such as network administrator or system administrator.null IP address are traditionally written in dotted decimal notation in which the bytes of the address are separated by periods and each byte is expressed as an integer represented in traditional base ten notation. null For example, using dotted decimal notation, the pattern 5.2 would represent the two-byte bit pattern 0000010100000010, which consists of the byte 00000101 (represented by 5) followed by the byte 00000010 (represented by 2), and the pattern 17.12.25 would represent the three-byte bit pattern consisting of the byte 00010001 (which is 17 written in binary notation), followed by the byte 00001100 (12 written in binary), followed by the byte 00011001 (25 written in binary).null Thus a computer in the domain of the Addison-Wesley publishing company might have the IP address 192.207.177.133, where the first three bytes (192.207.177) form the network identifier (identifying the Addison-Wesley domain) and the last byte (133) is the host address (identifying a particular computer within Addison-Wesley’s domain).null Addresses in bit-pattern form (even when compressed using dotted decimal notation) are rarely conductive to human consumption. For this reason each domain is also assigned a unique mnemonic address known as a domain name. null For example, the domain name of the Addison-Wesley publishing company is aw.com. Note that the naming system reflects the domain’s classification, which in this case is commercial as indicated by the com suffix. Such classification is called a top-level domain (TLD).null There are numerous TLDs, including edu for educational institutions, gov for U.S. government institutions, org for nonprofit organizations, museum for museums info for unrestricted use, and net, which was originally intended for Internet service providers but is now used on a much broader scale. null In addition to these general TLDs, there are also two-letter TLDs for specific countries (called country-code TLDs) such as au for Australia and ca for Canada.null Once a domain has a mnemonic name, its local authority is free to extend it to obtain mnemonic manes for the machines within its domain. For example, an individual machine within the domain aw.com might be identified as ssenterprise.aw.com.null We should emphasize that the dotted notation used in mnemonic addresses is not related to the dotted decimal notation used to represent IP addresses. Instead, the sections in a mnemonic address identify the computer’s location within a hierarchical classification system. null In particular, the address ssenterprise.aw.com indicates that the computer known as ssenterprise is within the institution aw within the class (or TLD) of commercial domain com. In the case of large domains, a local authority might break its domain in to subdomains, in which case the mnemonic addresses of the computers within the domain might be longer. null For example, suppose Nowhere University was assigned the domain name nowhereu. edu and chose to divide its domain into subdomains. Then, a computer at Nowhere University could have an address such as r2d2.compsc.nowhereu.edu, meaning that the computer r2d2 is in the subdomain compsc within the domain nowhereu within the class of educational domains edu.null Each domain’s local authority is responsible for maintaining a directory containing the mnemonic address and the corresponding IP address of those computers within its domain. This directory is implemented on a designated computer within the domain in the form of a server, called a name server, that responds to requests regarding address information. null Together, all the name servers throughout the Internet are part of an Internet-wide directory system known as the domain name system (DNS) that is used to convert addresses in mnemonic form into their equivalent bit-pattern forms. null In particular, when a human requests that a message be sent to a destination identified in mnemonic form, DNS is used to convert that mnemonic address into its equivalent IP address that is compatible with the Internet software. The process of extracting information from DNS is often referred to as a “DNS lookup.” Normally, a DNS lookup is completed in a fraction of a second.