The Internet is a collection of connected networks that communicate with a common protocol and provide a collection of services including electronic mail, remote login, file transfer (ftp), gopher, usenet newsgroups, and access to the World Wide Web.
Note: The Internet is not a collection of phone lines!
Client/server model - The client asks for information and the server does some processing to produce the results and give them to the client. There are typically many, many clients that use the same server. The clients primarily display the results, including possibly graphics and/or sound, on the local machine.
When you use Netscape, Eudora, and FTP, you are using the client software. The clients are connecting to servers on remote computers (hosts) to provide you with the information you requested. Server software is very complex and must be set up by the system administrator.
File server - A file server is a large computer with fast input/output capability and a huge amount of disk space. The file server keeps the "original" copies of files for other machines, and "serves" them when requested to do so. It does not process the files, but produces a copy when asked. This means that you can log into any one of a group of computers and still have access to the same files. You may edit the files locally, but when you save the files, they are saved back on the file server's disk, so they are available in the future from the server.
You have already learned what the Internet is, and used many of the services provided by the Internet. But how can such services be provided over the great distances (across the world) and between millions of different computers and in hundreds of different languages?
Computers communicate with each other across the world in the following manner. Each computer (host) must have its own unique "address". Computers use only numbers, and each machine has its own unique address. For humans, we use names like "bullwinkle.ucdavis.edu", to distinguish between computers. The translation between the numeric address and the "name" is done whenever necessary (described below).
Internet addresses (numeric) consist of four numbers (each between 0-255 inclusive (8 bits)) separated by "dots". The numbers to the left in each address indicate the company or institution that owns the host. The entire number is unique to that machine, with the rightmost number being the ones that change within the same organization or company. IP (Internet) addresses are assigned to organizations or companies by a standard organization. The organization then decides which machines to have use each number. When you dial in and connect to the campus with a SLIP (Serial Line Internet Protocol) or PPP (Point to Point Protocol) protocol, you are assigned a "temporary" IP address and name for the time you are connected. When you disconnect, someone else will get that address and name.
A name server is a software program that translates "human readable" computer names into their numeric addresses that computers respond to (and also the reverse). In the English addresses, the specific machine's name is on the left, followed by the department, the institution/company, and the type of institution or company.
The table below shows some example internet (IP) addresses including the "dot" address on the left and the name on the right:
IP Address Name ----------- ------------------ 128.120.8.149 dale.ucdavis.edu dale (2 of the IT machines you use for email) 128.120.8.148 chip.ucdavis.edu chip 128.120.56.188 toadflax.cs.ucdavis.edu (in the computer science dept at UCD) 128.120.4.18 gauguin.ucdavis.edu (machines in the UCD visualization lab) 128.120.4.15 cassatt.ucdavis.edu 128.101.149.1 cs.umn.edu (in the CS dept - U of Minnesota) 134.84.132.2 boombox.micro.umn.edu (the machine where "Gopher" lives) 128.178.155.13 liasun4.epfl.ch (in a research lab in Switzerland)
In the US, the last word in the English host name indicates the
type of institution.
Some examples are:
.GOV - government
.MIL - military
.EDU - educational institutions
.COM - companies
.NET - networking/communications organizations
.ORG - non-profit organizations
In the rest of the world, the last part of the host name is used to
indicate the country. Some examples are:
.CH - Switzerland
.JP - Japan
.CA - Canada
.SE - Sweden
Some of the terms we have been using in our discussions of the Internet and related services are:
Hypertext -- Hyperlinks are links that can go between different documents. The documents on the Web are called pages. Each page can have links that connect to other pages, anywhere on the web (including across the world).
http stands for hypertext transfer protocol - the protocol used to send hypertext documents between different computers. html stands for hypertext markup language - the language most web documents are written in (including this very page).
URL - A uniform resource locator. These contain the name of
the transfer protocol (http is an example), the name of the server
(www.cs.ucdavis.edu is an example), and the path up to the file name
(~reedn/ecs15/homepage.html is an example) in the following
format:
< protocol > :// < server > / < path > < filename >
Example: The ECS15 Course Home Page is located at:
http://wwwcsif.cs.ucdavis.edu/~reedn/ecs15/
The file name in this
case defaults to homepage.html when not specified. If there is no
homepage (or default page), a directory listing of the contents of the
directory is usually displayed.
A protocol is a set of rules for the exchange of data between a terminal and a computer or between two computers. The Internet uses the TCP/IP protocol (Transmission Control Protocol/ Internet Protocol). TCP/IP can be considered the "glue" that holds the Internet together. It allows computers with greatly differing hardware, operating systems, languages, and other software to communicate with each other.
The Internet is used to move information from place to place, all over the world. Internet services include:
FTP allows users to copy files from one machine to another. The capability called anonymous FTP is particularly useful. This means that you can copy files from or to a computer without having a personal account on that computer - you can log in as "anonymous".
Archie is a search program that can be used to find ftp sites where specific file(s) are available.
Veronica is a search engine for gopherspace - it can be used to find gopher sites with specific information.
Newsgroups (also called usenet newsgroups) contain a collection of messages written by Internet users and grouped by subject area. Each message is called an article. Newsgroups are accessed by a newsreader program. Netscape includes a newsreader facility. Other newsreaders are tin (text mode similar to pine under Unix), rn (text mode under unix), and xrn (graphical interface under Unix and X-windows).
The reader selects newsgroups on topics that may interest them and then read any of the articles posted under that topic. The reader may also post (create a new) articles. There are thousands of newsgroups, some on technical topics, others purely recreational in nature.
For this class, we have two newsgroups set up for our use at UCDavis: ucd.class.ecs15 (for the staff to post announcements to the class) and ucd.class.ecs15.d (for students to post questions of others, including other students and the staff). There are also newsgroups for programming languages, like Scheme (an example is comp.lang.scheme).
Note that the ECS15AT course has a separate newsgroup ucd.class.ecs15at for their use.
See pages 9-10 of Netscape At a Glance for more about newsgroups.
The World-Wide-Web (WWW) is a network-based information service that uses the Internet. Each file available on the web is called a page (even though it may require many pages to actually print the information). Information on "the web" is accessed with a browser. Netscape (Lab 4) and Mosaic are the most common graphical browsers and they are available for a wide variety of computers. Lynx is a text-based web browser particularly useful if you are dialing up with a modem, but do not have a SLIP connection and graphical browser software. With Lynx, you will see all the text and can follow links, but you will not be able to directly view any graphical images on the pages you visit.
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lectures for ECS15 - Fall 1997 .
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