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. 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.
The Internet cloud
自治系统 AS
R1 网1 R2 网3 网2 R3
主干区域 0.0.0.0
R6 R5 R4 网5 R8 网4 R7 网7 网8 网6 R9
区域 0.0.0.1
区域 0.0.0.2
区域 0.0.0.3
Ⅱ. Conneimplify 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. 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.
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. 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.
Most of this original work was sponsored by the U.S. government through the Defense Advanced Research Projects Agency (DARPApronounced 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.
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.
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. 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.
This temporary connection model is popular (and, in fact, expanding) in the cellphone/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. 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.
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.
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.