Internet basics
What is the World Wide Web's place in the Internet?
World Wide Web (WWW or Web) is one facet of the Internet consisting of client and server computers handling multimedia documents. Client computers use browser software (such as Netscape Navigator) to view documents (pages). Server computers use server software to maintain documents for clients to access.
Web documents are created by authors using a language called HTML (HyperText Markup Language) that offers short codes (also called tags) to designate graphical elements and links. Clicking on links brings documents located on a server to a browser, irrespective of the server's geographic location. Documents may contain text, images, sounds, movies, or a combination.
Documents are addressed with a URL (Uniform Resource Locator or, for short, location). Clients and servers use a document's URL to find and distinguish among documents.
What does each part of an Internet address mean?
Here's the components of the Internet address [email protected]:
The three-letter suffix in the location name helps identify the kind of organization operating the server. (Some locations use a two-letter geographical suffix.) Here are the common suffixes and organizational affiliation:
What's the purpose of server push and client pull?
Generally, pages are brought to your screen as a result of your input. You click on a link or an image to request a page. But servers also have the ability to automatically deliver an updated version of a page to your screen. For example, a weather watcher might want to see an updated satellite photo at 15-minute intervals. Netscape provides the pages creators and server administrators two complementary tools for making this work.
In server push, a HTTP connection is held open for an indefinite period of time (until the server is finished sending data to the client or until the client interrupts the connection). In client pull, a HTTP connection is never held open; rather, the client is told when to open a new connection and what data to fetch. Server push uses a variant of the MIME message format "multipart/mixed" that lets a single message (or HTTP response) contain many data items. Client pull uses an HTTP response header (or equivalent HTML tag) that tells the client what to do after a specified time delay.
How do I learn to write HTML and publish pages?
You can write HTML (HyperText Markup Language) pages using any word processor or text editor. To publish pages on the Internet, you need to submit your pages to a server (a networked computer running server software). HTML uses embedded codes (tags) to designate graphical elements and links. These codes can be produced simply from your keyboard using angled brackets and the slash character. For example, the tag <B> presents text in bold letters. An HTML source file containing the expression <B>This stands out.</B> is displayed on screen in bold. Notice that the tag </B> is required to notate the end of the bold expression.
HTML consists of many such tags, including tags for big headlines, underlining, italics, titles, paragraph breaks, images, and links. Browsers supports tags that allow you to create frames, tables, custom backgrounds, and JavaScript programming code.
One feature you'll certainly want to use is the HTML link. Here's an example of HTML that creates a link for users to click on:
<A HREF="http://www.aimss.net/">AIMSS.NET</A>
The section between quotes is the page's URL you'll see when you click on the link. The text following the URL contains the highlighted text (Home) the user sees on screen. The tag coding and brackets are also a required part of the link.
The method to insert images in your pages in very similar to that of inserting links. Inline images are links to image files. To see any one page's HTML source, you can choose the View|Document Source menu item or View Source. This command displays the text and tags used to create the content and content style of the current page.
How can I save files and images onto my hard disk?
You can also save a page to disk without displaying the page onscreen. Position the mouse over a link or image, then click the right mouse button to produce a pop-up menu with the items Save this Link as and Save this Image as for saving a file.
On Windows, you'll need to select "All Files" for GIF, JPEG, or other nontext files to show up in the Open File dialog.
What is dithering and how is it used?
Dithering is a software imaging process for arranging adjacent pixels of varying shades to achieve a visual effect. The process often enhances a computer's ability to display an image, and is particularly useful when the color or resolution of the original image must be reproduced on computers with different display capabilities.
Display of inline GIF and JPEG sometimes needs to translate an image's colors into similar colors available on your computer. If your computer doesn't have color capabilities that match the color information in an image, the image might look speckled.
JPEG images, however, are always dithered.
The default choice, Automatic, lets Netscape choose the image display determined to be most appropriate. When you choose to Dither, Netscape dithers the computer's available colors to most closely match the image's colors. When you choose Substitute Colors, Netscape substitutes an available color that most closely matches the image's colors. Images that are displayed with dithering take slightly more time to display than images using color substitution.
Plug-in modules are software programs that extend the capabilities of Netscape. A plug-in is installed on your hard disk using instructions that come with the plug-in. After installation, Netscape uses the plug-in's capabilities like other built-in Netscape features.
The plug-in application programming interface (API) allows third parties to extend Netscape with native support for new data types and additional features. Plug-ins are dynamic code modules, native to each Netscape platform. Plug-ins are complementary to architectures such as OLE and platform-independent programming languages such as Java. Here are the primary goals of the plug-in API:
The Netscape user interface remains relatively unchanged even when plug-ins are in use. Frames without plug-in data function like ordinary frames. Basic operations such as navigation, history, and opening files are not changed by plug-ins.
A plug-in can retrieve a URL with the same network functionality as Netscape. The data from such a URL is provided as a stream as the data arrives from the network. Plug-ins can themselves generate data that Netscape or other plug-ins can display. Plug-ins can both produce and consume data.
Plug-ins are associated with a MIME data type that Netscape does not natively support. When Netscape encounters an unknown data type from a server, Netscape looks for a plug-in that is associated with that MIME type and loads the plug-in.
What is a cache and how does it work?
A cache temporarily stores the information on a page in your computer. The first time you ask for a page, Netscape retrieves the page from the network. No pages are permanently stored in a cache. If you request a page you have seen before, Netscape checks to see if the page is available in a cache. For example, if you use the Back button to display a page, a cache can display the page more quickly than the network can retransmit it.
Sometimes you might not want a page to be retrieved from a cache. The page you displayed initially may be different than the page currently offered by the network. If a modification to a particular URL has occurred, you may want the updated page rather than the copy (now stale) stored in a cache. Remember, you have no control over when a server updates its pages.
When you click on a link, choose a bookmark, enter a URL, or press the Reload button, Netscape checks with the server to see if an update has occurred before bringing a page from a cache. If any change to the page has occurred, a fresh version is transmitted over the network; otherwise, a copy is quickly retrieved from a cache.
If you press the Reload button while holding down the Shift key (Option key on Macintosh), Netscape retrieves a fresh version from the network regardless of whether the page has been updated. The cache is not used. This type of reload is useful if you suspect the cached copy of a page has been corrupted.
When you press the Back button or choose a history item, Netscape does not check the network. Since you are explicitly requesting a previously viewed page, Netscape tries first to retrieve the cached copy (if still present in the cache) even if the server offers a more recent version.
What is the difference between memory cache and disk cache?
Netscape supports memory and disk cache to improve performance and reduce network traffic. When you bring a page from the network, information is stored in both caches. Netscape retrieves a page from the memory cache more quickly than from the disk cache, though retrieving from the disk cache is still faster than fetching from the network.
The disk cache has the advantage of persistence. When you exit a session (quit the Netscape application), the memory cache is emptied, but the disk cache is maintained (and takes up space on your hard disk).
Sometimes a cache can get confused (such as when servers provide inaccurate page modification dates). If you suspect a cache is acting improperly (such as providing stale pages) or just wish to free up space, you can clear the caches by pressing the Clear Memory Cache Now and Clear Disk Cache Now buttons in the Network|Cache panel (Clear Disk Cache Now is only on Macintosh).
What is the recommended size for each cache?
You can change the size of each cache to maximize its effectiveness. A larger cache might increase Netscape performance, though allocating too much space can constrict other applications. You might try to increase the size of the memory cache to whatever your system routinely has unused and increase the disk cache to between 2,000 and 5,000 kilobytes (2 and 5 megabytes).
You might find that a large disk cache increases the time required for the Netscape application to quit. If Netscape's cache maintenance causes undue delay when you exit the program, consider reducing size of the disk cache.
On Windows and UNIX, cache size and location preferences are set in the Network|Cache panel. An editable field in the panel allows you to set the directory location of the disk cache.
How can I access the Internet?
Some companies maintain a network that is linked to the Internet via dedicated communication lines. Those with less substantial resources, including most individuals, access the Internet via a service provider. A service provider is a company that offers use of its dedicated communication lines. If you have a modem, you can dial up a service provider whose computers will connect you to the Internet, typically for a fee. Dial-up access means that the modem on your computer can log in to another computer that is hooked up to the Internet.
The most popular dial-up access alternatives are shell accounts and SLIP/PPP accounts. When using a shell account, you dial into a service provider's computer and use the UNIX operating system to indirectly connect to the Internet. With an indirect connection, your computer does not interact with Internet computers. In many cases, when you download a file from an Internet site, the file is saved on the service provider's computer rather than on your computer. You then have to transfer the file from the service provider's computer to your home system. Shell accounts, while limited in features, have historically been less expensive than direct access accounts.
When using a SLIP or PPP account, you dial into a service provider's computer and run applications that directly connect to the Internet. With a direct connection, your computer can use browsers with user-friendly graphical interfaces to interact with Internet computers. A direct connection lets you download files directly to your system from remote sites. SLIP or PPP access to the Internet offers more performance and convenience than a shell account.
SLIP, short for Serial Line Internet Protocol, and PPP, short for Point-to-Point Protocol, are Internet standards for transmitting Internet Protocol (IP) packets over serial lines (phone lines). Internet information is packaged into IP packets, a method for enclosing data into small, transmittable units (wrapped up on one end, unbundled on the other).
A service provider might offer SLIP, PPP, or both. Your computer must use connection software (usually provided by the service provider) that matches the protocol of the server's connection software. PPP is a more recent and robust protocol than SLIP.