Network Sales and Services Handbook (Cisco Press Networking Technology)

"The Web," is a system of Internet servers interconnected across the world. The WWW provides a single interface for accessing Internet services, such as e-mail and file sharing.

The operation of the Web depends on hypertext documents, or pages, as the primary means of information retrieval. Hypertext is a document containing words connecting, or linking, to other documents. In the context of the Web, words or graphics can serve as links to other documents, images, video, and sound.

Web pages are created using the HTML. With HTML, tags are placed within the text to format the document with visual features such as font size, italics, bold text and to create the hypertext links, using either text or graphics as the link's anchor (the source of the hypertext link). HTML is an evolving language, with each code release bringing new HTML tags enabling additional features for programmers to use when designing web pages.

In addition to hyperlinked documents, or pages, the Web provides multimedia presentations, real-time collaboration, interactive pages, radio and television broadcasts, and the automatic push of information to a client computer. Programming languages such as Java, JavaScript, and Visual Basic are extending the capabilities of the Web. An increasing amount of information on the Web is served dynamically from content stored in databases. The Web is therefore not a fixed entity, but one that is in a constant state of flux.

The Web provides a single interface for accessing all these protocols. This creates a convenient and user-friendly environment. It is no longer necessary to be conversant in these protocols within separate, command-level environments. The Web gathers these protocols into a single system. Because of this feature, and because of the Web's capability to work with multimedia and advanced programming languages, the Web is the fastest-growing component of the Internet.

DNS

At the heart of the Web is the Domain Name Service (DNS), often referred to as DNS. DNS maps the names to Internet Protocol (IP) addresses for servers, such as web hosts, using the TCP/IP protocol.

All TCP/IP hosts (clients and servers) have an individual address, or IP number, such as 150.203.2.15 or 203.10.76.34. This IP address is used when an end-user connects to another computer also using TCP/IP. Because these numbers can be difficult for humans to remember each machine also has an individual name, such as www.cisco.com or www.whitehouse.gov. DNS is used to match up these names to their IP address. Figure 20-2 illustrates the DNS operation.

The following steps detail how DNS operates from an end-user perspective:

1. The web browser (client) queries the DNS server asking "What is the IP address for www.cisco.com?"

2. The DNS server looks in its database and finds the associated IP address to www.cisco.com: 198.133.219.25. The DNS server then sends this learned IP address to the requesting client.

3. The web browser (client) establishes an IP connection to the IP address learned from the DNS query.

Internet and WWW Applications

Almost every protocol type available on the Internet is accessible via the Web. Internet protocols are rule sets that enable communication between hosts across the Internet.

The following major protocols are accessible via the Web:

• E-mail Uses the Simple Mail Transport Protocol (SMTP) to distribute electronic messages and files to one or more electronic mailboxes.

• Telnet Facilitates login to a remote computer host to execute commands.

• FTP Uses the File Transfer Protocol (FTP) to transfer text or binary files between an FTP server and client.

• Usenet Uses the Network News Transfer Protocol (NNTP) to distribute Usenet news articles originating from newsgroup discussion boards.

• HTTP Uses the HyperText Transfer Protocol (HTTP) to send and receive hypertext across the Internet.

• Chat and Instant Messaging Uses several vendor specific protocols such as America Online (AOL) and Microsoft enabling Internet users to communicate with each other in real time by typing in a chat or instant messaging client.

Each of these protocols is discussed in the following sections.

E-mail

E-mail enables computer users to exchange messages across the Internet. Each e-mail user has a mailbox address to which messages are sent and received.

E-mail enables users to send and receive electronic files; these electronic files are referred to as Multimedia Internet Mail Extension (MIME) attachments. MIME was developed to enable e-mail software to handle a variety of file types. For example, a document created in Microsoft Word can be attached to an e-mail message and retrieved by the recipient, using the appropriate e-mail program.

Telnet

Telnet is a program enabling a user to log onto a computer on the Internet, such as an online database, library catalog, or chat services. To Telnet to a computer, the end-user must know its address, either the common name address (locis.loc.gov) or its IP address (140.147.254.3).

The most common web-based resources available through Telnet are library catalogs. For example, the Ohio State University library catalog can be reached via telnet by entering the following command at a command prompt: telnet library.ohio-state.edu. A link to a Telnet resource often looks like any other command-line link, the difference being that a Telnet session is launched to establish the connection to the remote resource.

A Telnet client is required on the local host (computer) in order for the user to establish a Telnet session to a remote resource. Most operating systems, such as Unix, Linux, and Microsoft, include a Telnet client as part of the operating system (OS). For example, a Microsoft Windows user can access the Telnet client through a MS-DOS command prompt window by typing "telnet."

FTP

FTP is both a program and the protocol used to transfer files between computers. Anonymous FTP is an option enabling users to transfer files from host computers on the Internet to their personal computers without having to establish user accounts on the remote host. For example, anonymous FTP sites can house books, articles, software, games, images, sounds, multimedia, course work, and so on, for users to download to their computer for later review.

An FTP client is required on the local host (computer) in order for the user to access files from an FTP server. Most operating systems such as Unix, Linux, and Microsoft Windows include an FTP client as part of the OS package, which can be accessed with the command, "ftp." FTP transfers also can be done with a web browser, using the command "ftp" in the browser window rather than the normal "http" used when accessing a web site.

For example:

• The command http://www.cisco.com instructs the web browser to establish a connection with the web server using the HTTP.

• The command ftp://ftp.cisco.com instructs the web browser to establish an FTP connection with Cisco.com's FTP server.

Usenet News and E-Mail Discussion Groups

Usenet News is a global electronic bulletin board system where computer users can exchange information regarding any topic. The major difference between Usenet News and e-mail discussion groups is that Usenet messages are stored on central computers, and users connect to these computers to read or download the messages posted to newsgroups. The distinction between Usenet and e-mail distribution is that with e-mail, messages are sent to an electronic mailbox for each user; newsgroup messages are sent to a central server for users to read and/or download.

Usenet itself is a set of machines that exchanges messages, or articles, from Usenet discussion forums, called newsgroups. Usenet administrators control their own sites and decide which newsgroups to sponsor and which remote newsgroups to allow as part of the system.

Chat and Instant Messaging

Chat programs enable users on the Internet to communicate with each other in real time by typing in a chat or instant messaging client. Chat is sometimes included as a feature of a web site, where users can log into a "chat room" to exchange comments about a particular topic.

Internet Relay Chat (IRC) is a service that allows users to communicate with each other on hundreds of channels based on specific topics. To access IRC, an IRC software program must be used.

A variation of chat is instant messaging (IM), where Internet users can communicate with other users on the same IM platform, such as America Online's Instant Messenger (AIM), Microsoft MSN Messenger (MSN), or ICQ.