rnet as marking, or as a service to their customers. A growing number of companies provide for –pay services over the Internet, as well. You can retrieve information with other networks, too, but usually only more slowly, often only through mail, sometimes only from one machine, and usually less conveniently.
You can connect across the Internet to a computer in another room, department, or country, and log in there as if you were directly connected. With proper permission on each end, you can use this service to access supercomputers or to log into your own office computer from your home or while you are travelling. The Internet is a general purpose network, with multiple services multiplexed over the same links for multiple users.
The concept of distributed resource sharing is new to many personal computer or mainframe users. You don’t need a wheelbarrow to carry floppy disks around to isolated PCs anymore. Instead, you can make the information available over the Internet, where anyone who needs it can get it.
3. Resource Discovery
Finding an appropriate file, document, host, or person among millions of hosts and many gigabytes of public repositories can be a problem. Fortunately, Internet resource discovery services have been developed in recent years to help solve this problem. Internet resource discovery services include:
. An online index of thousands of repositories
. Full-document keyword lookup to thousands of other repositories
. Hundreds of major library 无忧论文 【http://www.uklunwen.com】catalogs, including the Library of Congress and many major university catalogs
. Online bookstores
. Dozens of specialized databases
4. The Community
People use the Internet to get free software, to communicate with each other, to use supercomputers and databases, and for many other purposes. The capabilities, speed, and variety of Internet services and resources, although hard to explain to non-users, ensure that few people ever leave the Internet once they try it[3].
5. Mail Architecture
Fig –1 shows the general architecture of a mail system. Such a system has two major parts: MTAs and Uas. A message transfer agent (MTA) routes a mail message towards its final destination by sending the message to another MTA. A user agent (UA) interacts with an end-user and allows the user to send and receive mail message. The arrows in the figure show the path a message might take when sent from a user using UA1 to a user using UA3.
The interaction between an MTA and a UA is mot defined in this general architecture. However, the usual paradigm is for the MTA to deposit a mail message in a user’s mailbox. A mailbox is a file that stores mail messages deposited by the local MTA that the user can then read
Fig-1 Mail System Architecture
Each mail message comprises two parts: the mail envelope and the envelope contents. Analogous to the recipient and return address found on a paper mail envelope, the mail envelope supplies the addressing information. |
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